To this general statement, however, an exception must be made in favour of certain Fungi, which require ready-made organic matter for their nourishment. There are also certain plants (such as the Sun-dew and the Venus' Fly-trap) which live to some extent upon animal food.

On the other hand, no known animal possesses the power of converting inorganic compounds into organic matter, but all, mediately or immediately, are dependent in this respect upon plants. All animals, as far as is certainly known, require ready-made proteinaceous matter for the maintenance of existence, and this they can only obtain in the first instance from plants. Animals, in fact, differ from plants in requiring as food complex organic bodies which they ultimately reduce to very much simpler inorganic bodies. The nutrition of animals is a process of oxidation or burning, and consists essentially in the conversion of the energy of the food into vital work; this conversion being effected by the passage of the food into living tissue. Plants, therefore, are the great manufacturers in nature, -animals are the great consumers.

There remain to be noticed two distinctions, broadly though not universally applicable, which are due to the nature of the food required respectively by animals and plants. In the first place, the food of all plants consists partly of gaseous matter, and partly of matter held in solution. They require, therefore, no special aperture for its admission, and no internal cavity for its reception. The food of almost all animals consists of solid particles, and they are therefore usually provided with a mouth and a distinct digestive cavity. Some animals, however, such as the tapeworm and the Gregarinæ, live entirely by the imbibition of organic fluids through the general surface of the body, and many have neither a distinct mouth nor stomach.

Secondly, plants decompose carbonic acid, retaining the carbon and setting free the oxygen, certain fungi forming an exception to this law. The reaction of plants upon the atmosphere is therefore characterised by the production of free oxygen. Animals, on the other hand, absorb oxygen and emit carbonic acid, so that their reaction upon the atmosphere is the reverse of that of plants, and is characterised by the production of carbonic acid.

Finally, it is worthy of notice that it is in their lower and not in their higher developments that the two kingdoms of organic nature approach one another. No difficulty is experienced in separating the higher animals from the higher plants, and, for these, universal laws can be laid down to which there is no exception. It might, not unnaturally, have been thought that the lowest classes of animals would exhibit most affinity to the highest plants, and that thus a gradual passage between the two kingdoms would be established. This is not the case, however. The lower animals are not allied to the higher plants, but to the lower; and it is in the very lowest members of the vegetable kingdom, or in the embryonic and immature forms of plants little higher in the scale, that we find such a decided animal gift as the power of independent locomotion. It is also in the less highly organised and less specialised forms of plants that we find

the chief departures from the great laws of vegetable life, the deviation being in the direction of the laws of animal life.

6. MORPHOLOGY AND PHYSIOLOGY.

The next point which demands notice relates to the nature of the differences between one animal and another, and the question is one of the highest importance. Every animalas every plant-may be regarded from two totally distinct, and, indeed, often apparently opposite, points of view. From the first point of view we have to look simply to the laws, form, and arrangement of the structures of the organism; in short, to its external shape and internal structure. This constitutes the science of morphology (poppy, form, and λóyos, discourse). From the second, we have to study the vital actions performed by living beings and the functions discharged by the different parts of the organism. This constitutes the science of physiology.

A third department of zoology is concerned with the relations of the organism to the external conditions under which it is placed, constituting a division of the science to which the term "distribution" is applied.

Morphology, again, not only treats of the structure of living beings in their fully-developed condition (anatomy), but is also concerned with the changes through which every living being has to pass before it assumes its mature or adult characters (embryology or development). The term "histology" is further employed to designate that branch of morphology which is specially occupied with the investigation of minute or microscopical tissues.

Physiology treats of all the functions exercised by living bodies, or by the various definite parts or organs, of which most animals are composed. All these functions come under three heads:-1. Functions of Nutrition, divisible into functions of absorption and metamorphosis, comprising those functions which are necessary for the growth and maintenance of the organism. 2. Functions of Reproduction, whereby the perpetuation of the species is secured. 3. Functions of Correlation, comprising all those functions (such as sensation and voluntary motion) by which the external world is brought into relation with the organism, and the organism in turn reacts upon the external world.

Of these three, the functions of nutrition and reproduction are often collectively called the functions of organic or vegetative life, as being common to animals and plants; while the

functions of correlation are called the animal functions, as being more especially characteristic of, though not peculiar to, animals.

7. DIFFERENCES BETWEEN DIFFERENT ANIMALS.

All the innumerable differences which subsist between different animals may be classed under two heads, corresponding to the two aspects of every living being, morphological and physiological. One animal differs from another either morphologically, in the fundamental points of its structure; or physiologically, in the manner in which the vital functions of the organism are discharged. These constitute the only modes in which any one animal can differ from any other; and they may be considered respectively under the heads of Specialisation of Function and Morphological type.

a. Specialisation of Function.-All animals alike, whatever their structure may be, perform the three great physiological functions; that is to say, they all nourish themselves, reproduce their like, and have certain relations with the external world. They differ from one another physiologically in the manner in which these functions are performed. Indeed, it is only in the functions of correlation that it is possible that there should be any difference in the amount or perfection of the function performed by the organism, since nutrition and reproduction, as far as their results are concerned, are essentially the same in all animals. In the manner, however, in which the same results are brought about, great differences are observable in different animals. The nutrition of such a simple organism as the Amoeba is, indeed, performed perfectly, as far as the result to the animal itself is concerned-as perfectly as in the case of the highest animal-but it is performed with the simplest possible apparatus. It may, in fact, be said to be performed without any special apparatus, since any part of the surface of the body may be extemporised into a mouth, and there is no differentiated alimentary cavity. And not only is the nutritive apparatus of the simplest character, but the function itself is equally simple, and is entirely divested of those complexities and separations into secondary functions which characterise the process in the higher animals. It is the same, too, with the functions of reproduction and correlation; but this point will be more clearly brought out if we examine the method in which one of the three primary functions is performed in two or three examples. Nutrition, as the simplest of the functions, will best answer the purpose.

B

In the simpler Protozoa, such as the Amoeba, the process of nutrition consists essentially in the reception of food, its digestion within the body, the excretion of effete or indigestible matter, and the distribution of the nutritive fluid through the body. The first three portions of this process are effected without any special organs for the purpose, and for the last there is simply a rudimentary contractile cavity. Respiration, if it can be said to exist at all as a distinct function, is simply effected by the general surface of the body.

In a Coelenterate animal, such as a sea-anemone, the function of nutrition has not advanced much in complexity, but the means for its performance are somewhat more specialised. Permanent organs of prehension (tentacles) are present, there is a distinct mouth, and there is a persistent internal cavity for the reception of the food; but this is not shut off from the general cavity of the body, and there are no distinct circulatory or respiratory organs.

In a Mollusc, such as the oyster, nutrition is a much more complicated process. There is a distinct mouth, and an alimentary canal which is shut off from the general cavity of the body, and is provided with a separate aperture for the excretion of effete and indigestible matters. Digestion is performed by a distinct stomach with accessory glands; a special contractile cavity, or heart, is provided for the propulsion of the nutritive products of digestion through all parts of the organism; and the function of respiration is performed by complex organs specially adapted for the purpose.

It is not necessary here to follow out this comparison further. In still higher animals the function of nutrition becomes still further broken up into secondary functions, for the due performance of which special organs are provided, the complexity of the organism thus necessarily increasing pari passu with the complexity of the function. This gradual subdivision and elaboration is carried out equally with the other two physiological functions-viz., reproduction and correlation --and it constitutes what is technically called the "specialisation of function," though it has been more happily termed by Milne-Edwards "the principle of the physiological division of labour." It is needless, however, to remark that in the higher animals it is the functions of correlation which become most highly specialised-disproportionately so, indeed, when compared with the development of the nutritive and reproductive functions.

b. Morphological Type. The first point in which one. animal may differ from another is the degree to which the

principle of the physiological division of labour is carried. The second point in which one animal may differ from another is in its "morphological type;" that is to say, in the fundamental plan upon which it is constructed. By one not specially acquainted with the subject it might be readily imagined that each species or kind of animal was constructed upon a plan peculiar to itself and not shared by any other. This, however, is far from being the case; and it is now universally recognised that all the varied species of animals-however great the apparent amount of diversity amongst them—may be arranged under no more than half-a-dozen primary morphological types or plans of structure. Upon one or other of these five or six plans every known animal, whether living or extinct, is constructed. It follows from the limited number of primitive types or patterns, that great numbers of animals must agree with one another in their morphological type. It follows, also, that all so agreeing can differ from one another only in the sole remaining element of the question—namely, by the amount of specialisation of function which they exhibit. Every animal, therefore, as Professor Huxley has well expressed it, is the resultant of two tendencies, the one morphological, the other physiological.

The six types or plans of structure, upon one or other of which all known animals have been constructed, are technically called " sub-kingdoms," and are known by the names Protozoa, Colenterata, Annuloida, Annulosa, Mollusca, and Vertebrata. We have, then, to remember that every member of each of these primary divisions of the animal kingdom agrees with every other member of the same division in being formed upon a certain definite plan or type of structure, and differs from every other simply in the grade of its organisation, or, in other words, in the degree to which it exhibits specialisation of function.

VON BAER'S LAW OF DEVELOPMENT.-As the study of living beings in their adult condition shows us that the differences between those which are constructed upon the same morphological type depend upon the degree to which specialisation of function is carried, so the study of development teaches us that the changes undergone by any animal in passing from the embryonic to the mature condition are due to the same cause. All the members of any given sub-kingdom, when examined in their earliest embryonic condition, are found to present the same fundamental characters. As development proceeds, however, they diverge from one another with greater or less rapidity, until the adults ultimately become more or