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trary, I will merely remark that if you, or any of your numerous readers who may feel interested in this subject, will favour me with a visit to ray establishment, I shall be happy to give the fullest explanation as well as show the great difference existing between the two, will point out the cause of failure in their arrangement, and also the reason of the complete success of my own thermometer.

Though perhaps it is unfortunate for your correspondents that their reference to Dr. Miller was not made during his lifetime, yet, admitting that he said he was not aware of iheir arrangement, I must ask in all seriousness. What had their thermometer accomplished to make any one acquainted with it?

Facts speak for themselves. Their arrangement still remains without result, whilst my thermometer, which has solved the great problem of the true temperature of the sea even at its greatest depths, has been adopted not only by our own Government, but also by all the principal Governments and scientific authorities throughout the world. Louis P. Casella

147 Holborn Bars, Nov. 3

Squalus spinosus

Ox the 9th inst. the fishermen of Durgan, in Helford Harbour, sent for me to look at a fish new to them, which had been caught (with a \d. hook) on the preceding night near its entrance. Congers had been numerous, but suddenly ceased to bite. The fish (a spinous shark) had been hooked in the corner of its mouth, out of the reach of its sharp teeth, had wound the line many times round its body, which was 7 ft. in length, and 30 in. in girth, being longer and more slender than one of which I sent a notice to the Royal Cornwall Institution 38 years ago. The back, sprinkled over with spines, was of a dark grey colour, the belly nearly white. It was a male fish. The lobes of the liver were 4 ft in length. In the stomach was a partially digested dogfish, 2 ft. long. The upper lobe of the tail was muscular and long, perhaps to aid its ground feeding, the lower lobe more marked than in Dr. A. Smith's drawing, as given by Yarrel, and entirely unlike that of the Filey Bay specimen. Twelve hours or more after its capture, when all external signs of life had disappeared, I was surprised to observe the regular pulsations of the heart.

Prof. Huxley has not observed a correspondence between the mass and large convolutions of the brain of a porpoise and its intellectual power.

Several years ago a herd of porpoises wis scattered by a net, which I had got made, to enclose some of them. It was sttong enough to catch tigers if set in the straits of Singapore, across which they sometimes swim. The whole "sculle" was much alarmed, two were secured. I conclude that their companions retained a vivid remembrance of the sea-fight, as these cetacea, although frequent visitants in this harbour previously, and often watched for, were not seen in it again for two years or more.

Trebah, Falmouth, Oct. 27 C. Fox

Zodiacal Light

It is a matter for regret that with the magnificent opportunities of investigating the character of the Zodiacal Light afforded to Maxwell Hall by his elevated position in Jamaica, he does not seem to have brought the powers of either the spectroscope or polariscope to bear on it.

I think the full importance of the inquiry is hardly appreciated by many. Taking the generally accepted theory of the light— that of a lens-shaped disc of luminous matter, with the sun for its centre and a diameter exceeding that of the earth's orbit—its matter, lying as it does in the plane of the elliptic, actually connects us with the sun, and may be the medium through which the solar magnetic forces act upon our own.

The intimate connection between solar outbursts, auroras, and terrestrial magnetism is an established fact.

To the aurora, the zodiacal light is by many conceived to be nearly allied, and I do not think the evidence hitherto adduced against this theory is at all conclusive. The remarkable wave of light seen by Maxwell Hall is strongly in favour of it; and though spectroscopic observations seem to point the other way, they are as yet so scanty in number that it would be as unfair to argue from them the want of connection between the two phenomena, as it would be to assert that the planets have no volcanic fires of their own because they only give us a reflected solar spectrum.

Assume the zcxliacal light to consist of solid pan idea of sirter—planet dust—shining by reflected light, and it is not diSerf to imagine the aurora playing amongst these tiny world*, eicr. ■ which might have its own small magnetic system, swayed like a. own by the mister magnet, the sun.

So far as my own experience goes I can see no objection: . this assumption. Though I have seen the light very brillunu both its branches, I have never jret found it to have a deals outline. Nor have I been able to trace it either east or west-.) 1S00 from the sun. Granting that this can be done, however, ti. apparent vanishing point of the earth's shadow lies comptralit? near us, and far within this again is the point at which the sh«it» would subtend only a degree or two of arc, and at which it woii be very hard to discern mid the feeble light of this portion of f: zodiacal light; so that a slight extension of the diameter of •*• disc would remove any objection that might be raised under — • head.

Imagine one of Saturn's moons revolving in an orbit witii his belts, and fairly embedded in the matter, which, forties!' of the argument, we must assume to be illuminated by the plot To inhabitants of that satellite each night would bring apheKmenon closely resembling our zodiacal light, only far more taliant. At midnight two cones of light would taper up*in* east and west, and meet overhead. The brightest portionofeti cone would In: that along the axis and nearest the horizon. J wards the summit and on the borders, where the line ai sif&' would lie through less depths of matter, the light would S» dually fade away, but from the satellite being embedded in 1* belt, the entire sky would be more or less luminous.

Has it not been noticed on our earth that when the roiiitt light has been seen unusually bright, a "phosphorescence « the sky was everywhere visible? May this not arise (ma w solar belt in a somewhat similar manner?

From my personal observations I see no reason top"*1 lenticular form to the disc. Parallel faces would afford « p* spective such as the zodiacal light appears to me.

I would urge observers who may be fortunately situated, notB neglect opportunities. So far as I am able I shall do mf W to aid the work ot inquiry, and with the powerful instrument) that lirowning is forwarding me, placed at an elevation of more than 0,ooo ft., under the clear skies of our Indian wi iter, I tr»s I shall be able to add something to our knowledge of the zodiacal light.

I should feel much indebted to any of your reader!""? would inform me which is the best adapted polariscope for s"" researches, and whose (amongst makers) speciality such ins'raments are. L. H. Puisat-i

Camp Udapi, South Canara, Oct 3

Cold Treatment of Gases

Allow mc to submit to your readers the following sketch 0 an apparatus for producing extreme cold, by which it m'sTM perhaps be practicable to liquefy or even solidify the elementary gases which have hitherto resisted the efforts of chemists.

The gas to be operated on is compressed to any require'1 degree by means of one cylinder, is cooled to the lowest convenient degree in the ordinary way, passes into an expaasiM cylinder with a properly arranged cut-off, where in expansion it> temperature is still further lowered. From the expansion cylinder it returns back to the compression cylinder, extracting the l"'J' from the counter current proceeding from the compression cyl'nder, so that the latter will be always arriving at the expansion cylinder with a continually decreasing temperature.

As out here I have no possible means of trying whether there is anything in this idea, I offer it to any of your readers who may feel disposed to try it.

Graaff Reinet College, Cape Colony, T. GUTHM*

July 19.

•The Relation of Man to the Ice-sheet

Mr. Tiddeman his shown for Yorkshire what I proved "j years ago for the South of England in a paper in the oWcg** Magazine (vol. iv. p. 193), that glacial conditions have obtained in this country since its occupation by Palaeolithic man. Unw'' tunately an attempt which I made to explain this coincidence between his result and mine in a letter to the same periodical in February last was rendered abortive by a clerical (or perhop5 printer's) error. I would press upon geologists to considet

whether the point proved is not that a glacial period has intervened since the times of Palaeolithic man and the present, rather than that man existed in this country before the glacial epoch, I think Mr. Tiddeman thinks as I do; but I take the liberty ol stating this view more distinctly. O. Fisher

Wave Motion

In Nature, vol. viii. p. 506, Mr. Woodward has suggested a simple and ingenious illustration of wave motion. Could he, or any other correspondent, supply, or refer to, a popular explanation of the action of the particles upon each other, to which the propagation of the wave is due?

In the case of sound waves, the propagation is comparatively simple, and is fully and clearly explained in Dr. Tyndall's " Lectures on Sound," and elsewhere. Helmholtz, in his "Popular Lectures," has figured the motion of the individual particle* of which a water wave is composed. And in Sir John Herschel's "Familiar Lectures," there is an elaborate and beautiful demonstration of the motion of the particles of ether in plane and circularly polarised light; but neither of these expositions appears to deal with the mode of propagation of the motion by which the wave is formed.

On the other hand. Sir Charles Wheatstone's ingenious model beautifully exemplifies the interaction of vmz'ts and their results. But here the waves are produced by the wooden wave forms introduced into the machine, the beads representing the particles remaining fixed in relation to each other. Neither, therefore, can this explain the manner and direction of the actual impact of each particle upon the1 adjacent one (beginning with those in contact with the source of motion itself), to which, combined with the tendency to yield in the direction of least resistance, the water wave must owe its form, and upon which the still more complicated conception of the light wave must ultimately depend.

Could a reference be given to any practical explanation of this point, it would confer a l>enefit on many who are not competent to follow the subject into the higher mathematics. M. F. E.

Sussex, Nov.

Elementaty Biology

I, ALONG with many others, who are desirous of obtaining an insight into Nature, would esteem it a great favour, and it would be of the greatest benefit to us, if any of your scientific readers would undertake to give through your columns a short account of the various low forms of life included under the elementary stage of biology of the Science and Art Department. They might give instruction as to where the various objects could be seen, how inspected, names of the best text-books for the students' guidance, &c

By so doing, they would secure the praise of many who at present cannot find out the modes of studying such subjects.

Hull, Nov. S Biologv

Black Rain and Dew Ponds

CAn any of your readers explain the cause of this phenomenon? On Thursday, the 4th Sept., about 5 P.M., in the village of Marlsford, in the valley of the Thames, near Wallingford, a heavy storm of rain occurred : and the water which lell in several parts of the village was found to be nearly black. It is described as being of such a colour as would be produced by mixing ink with water. Another of these black water showers fell during the night of the following Friday.

Would any reader of Nature also kindly set forth the theory upon which the utility of the dew ponds, found in many of the highest points of the Berkshire Downs, rests. They are circular ponds made with considerable care, and are supposed to receive so much dew as to supply all the water needed fjr the sheep in their neighbourhood through the driest summer.

Tiverton E. HlGHTON

ALBANY HANCOCK

THE brief announcement by which some of our readers may have first learnt of the decease of one of our greatest biologists is, in its simplicity, in singular harmony with the life the close of which it commemorates.

The retrospect of so serene a career leaves little to the biographer, for its points seem marked rather by phases of study, as indicated by important scientific memoirs, than by incidents which the world regards as striking or noteworthy.

Albany Hancock was born at Newcastle-on-Tyne on Christmas Eve, 1806. His father, Mr. John Hancock, died some six years later, and of the six little children thus left dependent on their mother, Albany was the third. He received a good education as times then went, and on leaving school was articled to a solicitor of good standing in Newcastle. Uncongenial as was the employment, he served his full term, passed the customary examinations in London, and even took an office in Newcastle with the view of establishing himself in practice. But the occupation was irksome, and he gave it up ere long to join a manufacturing firm, and this in turn circumstances led him soon to abandon. The simple fact probably was that neither occupation permitted him to follow the bent of his inclination, and that the desk and counting-house were alike distasteful to a mind pre-engaged as was his by other currents of thought. His early taste for natural history pursuits was probably in part derived from the collections, chiefly conchological, formed by his father, who was in many ways a man of superior ability, and had been something of a naturalist; and association with the late Mr. Robertson and Mr. Wingate, the one a botanist, the other an ornithologist, of repute ; with the well-known Mr. Bewick; and abov; all with his near neighbour Mr. Alder, confirmed his inclination in this direction. He was. as a boy, clever with his fingers, and that manual dexterity which in later years served him so well when engaged with dissecting needle and pencil, exhibited itself in many of the pursuits of his early life.

The first mention we find of Mr. Hancock's devotion to natural history is in Mr. Alder's "Catalogue of Land and Fresh-water shells," published in 1830, in which the author handsomely acknowledges the obligations he is under to him and to Mr. John Thornhill "for the communication of many habitats observed during their active invesfgation of this as well as other branches of the natural history of the neighbourhood " of Newcastle. His earliest appearance as an author seems to have been in connection with two short papers in the first volume of "Jardine's Magazine of Zoology and Botany," published in 1836, the one a "Note on the Occurrence of Raniteps trifurcatus on the Northumberland Coast," the other a "Note on Fako rufipes, Regulus ignicapillus and Lai us minutus." These notices were, comparatively speaking, of trifling significance, but they were the beginning of a long series of contributions to knowledge which only ceased when his last illness deprived him of the power of continuous work. It is unnecessary here to enumerate the successive memoirs that embody the results of his life's labour. A catalogue of the original papers of which he was author, or joint author, would extend to something over seventy titles.

Early association with Mr. Alder in the study of the mollusca led to the production between the years 1845 and 1855 of their magnificent " Monograph of the British Nudibranchiate Mollusca," which may still be taken as a standard of excellence amongst such publications. Many of Mr. Hancock's earlier papers were devoted to the elucidation of the boring apparatus of the mollusca, and these were followed by similar researches respecting the excavating power of a group of sponges {Ciiona and allied genera) which until that time had been but little known or understood.

As an anatomist —and after all it was his large knowledge of minute anatomy and infinite skill in dissection that gave its especial value to most of his work—he was, perhaps, best known by his elaborate memoir on the Organisation of the Brachiopoda, published in the Philosophical Transactions for 1857; but many other papers of the same thorough and original character proceeded from his pen. Amongst them will be remembered the following :—" On the Olfactory Apparatus in the Bullidae" (1852); " On the Nervous Systems of Ommastrcphes todarus" (1852) ; "On the Anatomy and Physiology of the Dibranchiate Cephalopoda" (1861) ; "On the Structure and Homologies of the Renal Organ in the Nudibranchiate Mollusca" (1863); "On the Anatomy of Doridopsis" (1865) ; "On the Anatomy and Physiology of the Tunicata" (1867).

For some years previous to his death Mr. Hancock had devoted much attention to the fish of the Carboniferous period, and in conjunction firstly with Mr. T. Atthey, whose fine collection afforded ample material for the purpose, and subsequently with Mr. Howse, published a series of fifteen papers on these coal-measure fossils.

The promised Monograph of the British Tunicata, preparations for which had made some progress even before the death of Mr. Alder, had occupied much of his time; and though probably still unfinished, it may be hoped that the results of his investigations are so far complete in themselves, that the work, as far as it has gone, may be saved to science. A supplement to the Monograph of Nudibranchiate Mollusca had been a matter long on his miad, but one that he had never been able to devote himself to realising, beyond the collection of materials.

Allusion has been made to Mr. Alder, Mr. Atthey, and Mr. Howse, as having been associated with Mr. Hancock in certain of his papers; to these must be added the names of Dr. Embleton and the Rev. A. M. Norman as occasional colleagues.

On the establishment of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne in 1829, Mr. Hancock became an active supporter, and was one of the original staff of honorary curators; and on the formation of the Tyneside Naturalists' Field Club in 1846, he was one of its principal and most influential promoters. When the new College of Physical Science in Newcastle was instituted, his name, almost as a matter of course, was placed on the provisional committee; and it was only when this body had completed its labours and gave place to a permanent board, that he was permitted, on the ground of ill-health, to retire from active service in connection with the institution. He was a Fellow of the Linnean Society, a corresponding-member of the Zoological Society of London, an honorary member of the Imperial Botanico-Zoological Society of Vienna, and perhaps of some other similar bodies; but honours of this sort, though valued in their way, were thrust upon him rather than sought. Though living a retired life, no man more highly prized social intercourse. His kindly helping hand was held out to every young naturalist: Such were always welcome at his house; and when appealed to by them, as was often the case, he made their difficulties his own till he could help to solve them.

It is yet too soon to attempt to shake oneself free from a sense of his presence, or to essay to weigh in judicial balance the value of his contributions to human knowledge: considerations of this sort are overwhelmed in the sense of irreparable loss to science.

H. B. B.

FERTILISATION OF FLOWERS BY INSECTS*

IV.

On the tTvo forms of flower of Viola tricolor, and on their different mode of fertilisation.

T/'IOLA tricolor presents a further example of the same

kind of dimorphism as that described in the last article

in the case of Lysimahcia, Euphrasia, and Rhinanthus.

* Continued from vol. viii. p. 435.

One of its two forms, illustrated by Fig. 15 in natural sbe,o more conspicuous than the other (Fig. 16), not only by "J larger sire, but also by the more striking colour of :.i petals. When the flower has just opened, its two upre

Cetals are light violet, or, in rarer cases, nearly whits ut they gradually become a deep violet, or even in blue. Far more striking is, ordinarily, the char? of colour in the two lateral petals and the lower ox; which, immediately after the opening of the flower, ir nearly white, while in a fully-developed state tkj are always violet. The petals of the small-flower:, form of Viola tricolor, illustrated in natural sire by Fu_ 16, are, on the contrary, uniform in colour and nearly whit during the whole time of flowering. The attractiveness for insects of the two kinds must therefore be very ds j ferent, whereas those particular marks round the opening I of the flower which serve as a guide to insects in search: the honey, the" Saftmal" of Sprcngel, arc nearly the sunt in the two varieties. That part of the lower petal imrntdiately before the entrance of the flower (y, Fig. J1, £' is in both dark yellow, and the lower prtal is ak marked by black streaks converging towards the «aa: entrance. There is only this difference between the I" forms as to their guide-mark (Saftm^f), that in thelagtflowered form seven black streaks on the lower petti, ana three on each of the lateral ones point towards the entrant* | of the flower; whereas in the small-flowered form thes are but five black streaks in the lower petal, and noae J. all on the lateral ones.*

Although these two forms have been generally kno*n. at least since the time of Linrucus, all botanists who hiv; published observations on the fertilisation of V/nliitr,color have apparently turned their attention exclusively to the large-flowered form (Fig. 15), whose beautiful adaptations to cross-fertilisation by insects, have been, therefore. very accurately described; while the peculiarities >» structure and fertilisation of the small-flowered form bJ>'not even been mentioned. If, in this case, we clearly see that even scientific inquirers have been far more attracted by the larger violet flowers than by the staslk' whitish ones, we need not wonder that insects are in"u" enced in like manner, and that from this cause smaller and less conspicuous flowers are so frequently quite overlooked by insects, that they would rapidly become extinct, unless slight modifications of structure and development enabled them to produce seeds by self-fertilisation.

Indeed, in Viola tricolor, as in those species hitherto considered, regular self-fertilisation in the small-flowered form is effected by such slight modifications of structure and development, that by far the larger number of the contrivances in the large and small-flowered forms are identical.

In both forms, honey is secreted by two long appen" dages (n) of the lower filaments (fi), from which it ascefl<» by adhesion into the uppermost part of the hollo* spot (sf>); the style (sly, Fig. 22) is directed downwards on its base, slender and bent like a knee, while above it is straff*' and gradually thickened, but does not increase at all or only slightly in breadth, ending in a skull-like stigmau' knob (k), ihick enough to completely stop the entrance o< the flower. This knob is provided with a wide open moist stigmatic cavity (st,) and is protected from above by two sets of hairs (fir, Figs. 21, 22, Sprengel's "Sattdeckc ") °n the two lateral petals, which at the same time defend the entrance of the flower against rain, and prevent insects from entering into the flower in any other way than b)' the lower side of the skull-like knob. In both forms the five anthers open inwards, are narrowed towards thW

• My description relates exclusively to those varieties of Viola '"""J which grow in the environs of Lippstadt. r-'rom Sprengel's, Bennett's, aAa other descriptions and illustrations, 1 am aware that in other localities somewhat different varieties arc found. Hut I do not doubt that differences m the manner of fertilisation, identical or closely allied with those hereto 1* described, will be found wherever a large-flowered and a small-flowered ton11 of Viola tricolor co-exist.

end, and prolonged above into orange-coloured triangular appendages of their connectives (t-, Figs. 21, 22), and lie so close together round the style, as to form a hollow cone containing the pollen, and overtopped only by the skull-like crest of the style. This position of the stigmatic knob rising out of the anthercone but immediately below its summit, is secured by a remarkable contrivance, the skull-like knob being prevented from sliding into the anther-cone by two tufts of hairs, projecting like whiskers from its two cheek-like lateral surfaces. Thus a lifting up of the stigmatic knob, which must always be effected by insects seeking for honey or for pollen, and which is easily accomplished by them in consequence of the base of the style being slender and bent like a knee, will be more likely to tear off the filaments than to push the stigmatic knob into the anther-cone. Indeed, we find that by the swelling of the fertilised ovary the filaments are always torn off, whereas the anthers remain, enclosing like a hollow cone the narrow portion of the style, and the skull-like knob is never drawn between the anthers. If the anther-cone containing the pollen were densely closed all round, the pollen-grains would not fall out unless the anthers were separated from each other by lifting up the stigmatic knob ; but there actually exists an opening on the lower side of the summit of the cone directed downwards, the appendages of the two lower anthers being cut out [op. Figs. 21, 22), by which nearly all the pollen may fall out spontaneously. When it has fallen out, a great pan of the pollen is collected in the close hairy lining of the fore part of the spur.

Thus far the two forms of Viola tricolor are identical in structure; and the same, or nearly the same, insects may a priori be supposed and have really been observed, to visit the two forms. The distance between the closed entrance of the flower and the honey contained in the uppermost part of its spur being in both of the two forms 6-7 mm., an insect must be provided, in order to reach the honey, with a proboscis of at least that length, unless it be enabled by its small size to crawl with its whole body into the flower. A proboscis of 6-7 mm. length or larger is only to be met with among all our insects in Lepidoptera, Apidse, and some few Diptera; insects sufficiently minute to be able to crawl into and out of the flowers, are to be found chiefly in the genera Thrips and Meligethes. It may therefore be supposed, a priori, that Lepidoptera, Apidae, and Diptera provided with a proboscis of at least 6 mm. long, and very minute insects of the genera Thrips and Meligethes, will visit the two forms of Viola tricolor for honey, and that, besides, some other insects provided with shorter probosces will seek for their pollen. By direct observation this supposition has been thoroughly confirmed, as shown by the following list of visitors actually observed :—

I. As visitors of the large-flowered form, there have been observed :—(a) Lepidoptera: (1) Picris rapce L.* (l2).f—(&) Apida : (2l Bombns muscorum L.* (io-l 5); (3) B. lapidarius L.9J (12-14}; {s,)B.sp.%; (5) Anthophcra piUpcs F.!| (19-21}; (6) Atidrena albicans K. <J (2-2|),in vain seeking for honey.t—(c) Diptera : (7) Rhingia rostrata L.§ (n-12) ; (8) Syrilta pipiciis L (2-3), eating pollen.j—iff) Thysanoptera: (9) Thripr.^1

II. As visitors of the small-flowered form, there have

been observed :—(a) Lepidoptera: (1) Picris rapa L. ;* (12), repeatedly; (2) P. napi L.* (11), repeatedly; (3) Polyommatus Dorilis Hfh. *—(6) Apidae ; (4) Apis melliJiia L.(6) 8 ;t (5) Bombus hortoriim L.<J (18-21), perseveringly visiting the flowers for honey, although every flower isdrawn down by the weight of this large humble- bee;

(6) B. Rajellus FU.$* (10-13), the same individual visiting sometimes V. tricolor, sometimes Lamium purpureum;

(7) B. muscorum h.(agrorum F.) $ (10-14), visiting, without distinction, now the flowers of V. tricolor, now the nearly equally large and equally coloured flowers of Lithospermum arvense, while omitting the smaller ones of Capsella busra-pastoris, Valerianella olitoria, and Myosotis versicolor; (8) Osmia rufa L. <$ * (7-9), but once hastily visiting a flower for honey.—(c) Diptera; (9) Rhingia rostrata L.* (11-12), several specimens,repeatedly visiting flowers for honey.—[d) Coleoptera; (10) Meligethes* crawling into the flowers.

Direct observation has thus shown that no essential difference exists between the fertilisers of the large and those of the small-flowered form. But it must appear a striking fact that not only an equal number of different species, but even one more species has been observed on the small than on the large-flowered form. All the visitors of the small-flowered form, with the exception of only one, having been observed by myself, I must add, as an explanation of this fact, that I have repeatedly watched at the most favourable weather, for several hours, a neglected field, in which, besides some other weeds, there grew an abundance of vigorous specimens of the small-flowered form of Viola tricolor; whereas I have never had an opportunity of watching the large-flowered form under favourable conditions. Therefore I have no doubt that, in spite of the incomplete observations hitherto made on this subject, the more conspicuous flowers are in this species also really far more frequently visited by insects than the less conspicuous ones. Otherwise the differences in structure and development of the two forms now to be described would be quite inexplicable. These differences are:—1. In the largeflowered form the stigmatic cavity lies somewhat more towards the top of the skull-like end of the style than in the small-flowered one (as shown by the comparison of Fig. 17 with Fig. 18, and of Fig. 19 with Fig. 20.)

(1) When the skull-like knob in the two forms is pressed against the lower petal, in the large-flowered form the opening of the stigmatic cavity is directed outwards, so that pollen-grains which have fallen out of the anthercone spontaneously can never fall into the stigmatic cavity unless carried by insects; whereas in the smallflowered form the opening of the stigmatic cavity is directed inwards, so that pollen-grains falling out of tht anther-cone spontaneously, fall directly into the stigmatic cavity.

(2) In the large-flowered form the opening of the stigmatic cavity (st, Figs. 17, 19, 21) bears on its lower side, as discovered by Hildebrand, a labiate appendage (/, Figs. 17, 19, 21) provided with stigmatic papillae, so that a proboscis inserted into the flower, when charged with pollen of a previously visited flower, rubs off this pollen on to the stigmatic lip (/), thus regularly effecting cross-fertilisation ; whereas, when withdrawn out of the flower, charged with its pollen, the proboscis presses the lip (') against the stigmatic opening {st), thus preventing self-fertilisation. This nice adaptation to those visitors provided with a long proboscis (Lepidoptera, Apidae, Rhingia) is completely wanting in the small-flowered form (Figs. 18, 20, 22).

(3) In the large-flowered form there is a black wedgeshaped streak (u>, Figs. 17, 19) on the front side of the style, to which Mr. A. W. Bennett first called attention,* and which he has interpreted as a guide-mark for those visitors, which are diminutive enough to crawl entirely into the flower. This streak is also wanting in the small-flowered form (Figs. 18, 20).

• By W. E. Hirt (nature, vol viiL p. i»ij.

t The numbers enclosed between parentheses after the names of the insects indicate the length of their probosces in millimetres.

t By myseli (" Befruchtung tier Biumen dnrch Insecten," p. 145X

S By Ch- Darwin, who writes me, May 30, 1873:—" Between twenty and thirty years ago I observed, for two or three years, large beds (of V. tricoAVrJ in the flower-garden, and saw several times Rhingia rvs/mttt, and a nearly black humble-bee visit and fertilise the flowers. I say fertilise, because 1 bad watched the flowers for a l^ng time previously, and saw no insect visit them : but twu or three days after the above visits a multitude of flowers withered and set capsules."

1 Ey Dcipioo lM Ulteriori asservaiiooi," p. fin).

v By Sprengel (" Das emdedae Gchcitnniu," p. 7,7), and Mr. A.W. Benlett (N A rum*, vol. viii. p ao).

• By myself (June 1873).

t By Sprengel (h: cit.) and by myself (June 1873), perseveringly visiting the flowers for honey.

(4) In the large-flowered form pollen-grains do not spontaneously fall out of the anther-cone before the flower has been fully developed for several days ; whereas in the small-flowered form, in by far the majority of cases, a great number of pollen-grains lall spontaneously out of the anther-cone into the stigmatic cavity and there develop long pollen-tubes, even before the opening of the flower, in much rarer cases a short time after it has opened.

(5) When the visits of insects are prevented by a fine net, the flowers of the small-flowered form wither two or three

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Fig. 15.—Front view of the more conspicuous flower of Viola tricolor. natural size. Fig. 16.—Front view of the less conspicuous flower. Fig. 17.—Pistil of Fig. 15, viewed on the under side, 12 times natural size. Fig. 18.—Pistil of Fig. 16. Fig. 19 —Lateral view of the pistil of Fig 15. Fig. 20.—Lateral view of the pistil of Fig. 16. „

The following explanation of the lettering applies to all the figures :— a, anthers; a1, upper, a3, lateral, a*, lower anther; afi1, appendage of the upper sepal ; o, beard, 1 r. tuft of hairs on the lateral surface of the skull-like crest of the style : t", appendage of the connective ;Ji, filaments ; *, knob of the stigma ; /, lip, labiated appendage of the stigmatic opening; «, nectary, i.e. honey-sccrcting appendage of the lower filaments ; oft, opening of the anthercone ; ov, ovary ; /, petals; rf1, lower, p2, lateral, />J, upper petal ; /o, pollencollecting hairs : pr, protective hairs (Sprengel's "Safldecke ") ; t, sepals; r, upper sepal (with the appendage ajr); j-, lateral sepal ; sp, the uppermost part of the spur, containing the honey; st, stigmatic cavity; sir, streaks coovcrgine towards the opening of the flower; sty, style ; w, wedgeshaped streak of the style ; y, yellow Coloured part of the lower petal.

days after opening, everyone setting a vigorous seedcapsule ; those of the large-flowered form remain in full freshness more than two or three weeks, at length withering without having set any seed-capsule; when fertilised they wither also after two or three days.

Summary:—The more conspicuous flowers of Viola tricolor are adapted to regular cross-fertilisation by Lepidoptera, Apidae, and Rhingia ; whereas self-fertilisation by these visitors is prevented. Pollen-eating flies and

* In 'his interesting article on the Fertilisation of the Wild Pans)-, Natuse, vol. vm. p. 49.

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