It is evident from these considerations, that tropical Africa and America have borrowed little of one another within the period of the creation of the forms of plants now inhabiting each,that the differences between these Floras are so great, that it is doubtful whether at any time there has been much community of vegetation, and that the hypothetical modern Atlantic continent, which Heer assumes to have existed in the North Atlantic, and to have connected Europe and North America, cannot have extended to the south of the Tropic of Cancer.

If, on the other hand, we compare tropical Africa with tropical Asia, we find, 1, a vast amount of specific and generic identity; 2, an absence in Africa of any great or peculiar group, that is not also Asiatic; and 3, an absence in Africa of many of the great groups that are characteristic of Asia. The sum of these facts amounts to fair evidence, that tropical Africa was peopled by plants from tropical Asia, and that within a comparatively modern epoch. Up to the present time we have no sufficient data for comparing tropical Africa, generically even, with America beyond the West Indies, and until this is done, it would be rash to speculate upon the means whereby the few plants common to tropical Africa and the West Indies have been transported from the one to the other; or why it is that there should be so many Orders common to America and Asia, that are scantily represented, or totally absent in tropical Africa.

Turning now from these points of difference between the Floras of the Old and New Worlds to those of similarity, a comparison of the contents of Dr. Grisebach's Flora with those of Mr. Thwaites' enumeration, gives some curious results.

In the first place, the number of Natural Orders is almost precisely the same in both areas, viz., 156 in Ceylon, and 152 in the West Indies; and the Orderst themselves are to a great extent the same; the Orders not represented in both being, with the exception of six, either small or feebly represented. These are the following:

Present in the West Indies, but | Present in Ceylon, but absent in absent in Ceylon. the West Indies.

†The respective authors have slightly different opinions as to the limits of some of the Orders, but these are here reduced to the same standard.

Therefore the sum of the Ordinal differences between two spots in the tropics of the Old and New World respectively, and containing together upwards of 2000 genera and 5000 species is represented by only about 82 genera and 200 species.

If again we seek to ascertain the extent to which the dominant Orders are represented in each, we find a further great and remarkable uniformity. More than half the Flowering plants belong to eleven Orders in the case of the West Indies, and to ten in that of Ceylon, whilst with but one exception the Ceylon Orders are the same as the West Indian, and they follow in nearly the same sequence in each country.

The species in each column marked with an asterisk, are confined (in the

tropics) to the Old and New Worlds respectively.

Tropical Africa, according to the data published twenty years ago, in the Niger Flora, differs little in its ten dominant Orders, and their sequence from Ceylon; and lastly, to show that this uniformity is not accidental, we have taken the figures from Miquel's Flora of the Dutch East Indian Islands, which also includes a good many Continental Asiatic plants.

Again, of the 110 Orders, common to Ceylon and the British West Indies, only 37 are so unequally represented as to contain in one country double the number of species which the other contains. They are the following:

The following Orders are singularly equally represented in each :

It would be worth inquiring to what longitudes this similarity of vegetation is confined within the tropics. We know that no such Ordinal uniformity exists between the vegetation of extratropical Africa and South America, nor between that of either of these countries and extratropical Australia; and we also know that the Floras of the Mediterranean Region and the Southern American States, and those of middle Europe and the Northern American States, differ more than those of Ceylon and the West Indies in their Ordinal relations. This is a most interesting subject in relation to the hypothesis of an intertropical cold epoch, such as Mr. Darwin demands for the migration of the Northern Flora to the Southern hemisphere, and which epoch, occurring (as it must have occurred) since the creation of most of the existing temperate species, must have destroyed a great part of the pre-ex

istent vegetation of the Tropics, obliging us to regard the majority of existing tropical plants as modern creations compared with the temperate. With ourselves it is a matter of doubt whether the vegetation of the Tropics (exclusive of the temperate regions of its mountains) is richer generically and specifically than that of the Temperate zones. If it should prove to be richer, it presents a grave difficulty in the way of Mr. Darwin's hypothesis, and one which he may perhaps best obviate by showing that, owing to the struggle of life being greater in intertropical regions, and the greater activity of the subsidiary agencies (such as rapid development of vegetable life, insects and the elements that tend to hasten change), there has been a more rapid process of differentiation and selection. There will still, however, be the difficulty of accounting for the uniform distribution of Genera and Orders over the Tropics of the Old and New World, without any obvious means of transoceanic migration between them.

The last point to which we shall allude in the West Indian Flora is the astonishing number of Ferns it contains. Dr. Grisebach adopts Sir W. Hooker's moderate estimate of the value of specific characters, and according to it enumerates no less than 340 species (exclusive of 23 Lycopodiacea). Considering the area of the two islands which contain almost the whole of them, viz. Jamaica and Trinidad, this number is enormous. Ceylon, a very rich country, contains 205 species and 14 Lycopodiacea. Jamaica alone contains 290 Ferns and 17 Lycopodiaceœ.

In Mr. Thwaites' Enumeratio Plantarum Zeylaniæ, we have a work whose modest title covers a great amount of most accurate botanical matter, which will prove of the greatest use to the Indian as well as to the Cingalese Botanist. It comprises the Flowering plants and Ferns of one of the richest tropical islands in the world, perhaps the very richest, considering its area and elevation; and one which derives an additional interest from being the first the Flora of which was published in a systematic form, and this too by the great Linnæus.

The number of species described by Linnæus in 1757 as indigenous to Ceylon was 657. Mr. Thwaites enumerates no less than 2832, which might be considerably enlarged by those who take a narrower view of specific limits than he does. On this subject the author says in his preface," Care has been taken not to multiply species unnecessarily, for observation has shown that the amount of variation is often considerable in plants, affecting a large range of station, and