the life-history of the offspring would be a repetition of the life-history of the parent. Mr. Darwin thinks this sequence is ensured by the "affinity of these gemmules for other partially developed cells in due order of succession ;" and this appears to be an exceedingly probable explanation of many of the more complex phenomena of inheritance.

Gemmules which do not become developed may nevertheless multiply according to Mr. Darwin's hypothesis, and may be transmitted through many succeeding generations, thus enabling us to understand many remarkable cases of reversion or atavism, cases in which the lost or rather dormant characters of a more or less distant progenitor reappear in the progeny ; and the improbability, as Mr. Darwin has pointed out*, is not greater, that such rudimentary gemmules should be transmitted, than that useless rudimentary organs should be truly inherited through millions of generations.

*Origin of Species, p. 126.

NUTRITION is only a name for that peculiar series of chemical changes by means of which organic matter increases in bulk by the assimilation of new material. As we have already seen*, it is separated by no definite line from chemical change amongst inorganic bodies. Without change it is manifest that there could be no evolution; so there could be no progressive evolution without continuous change; and this condition is fulfilled by the nutritive process. Without it there could be neither growth nor reproduction; and, as we have seen, reproduction is the great means by which the animal and vegetable kingdoms have been evolved. It is therefore necessary that we should attain clear ideas concerning the relation of the function of nutrition and evolution.

The food-material of all organic matter is obviously unlike the matter nourished to a greater or less degree; even when, as in the case of fleshfeeding animals, it is very nearly like the organism which it nourishes, it undergoes changes in the * Page 23.

stomach which render it more or less unlike before it comes into contact with the tissues to be nourished. Again, material is required for the production of energy; and this is derived from food which is often of the same nature as that by which the organism itself is nourished; hence there is frequently, perhaps always, a balance of material which is excreted or thrown out from the part or organism. The food may therefore be divided into three parts, according to its ultimate destinationthat which nourishes the organism, that which is used for the production of energy, and that which is excreted or again turned out by the growing tissue.

In simple cellular organisms this excreted material often forms a shell around the growing tissue; and the same occurs almost universally in the vegetable kingdom. Plants, which expend but little energy, also secrete a large quantity of energy-producing material in the form of wood, cellulose, and sugar-the two former building the hard framework or skeleton which supports the delicate protoplasmic centres of nutrition by means of which the vital processes of the plant are carried on, and the latter being poured out as manna upon the leaves and branches or into the nectar-bearing organs of their flowers.

In the more complex forms of life, in which the

CORRELATED NUTRITION.

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food-material is distributed to various parts of the organism by means of the circulatory fluid, each cell assimilates to itself from this fluid certain definite constituents for its own nutrition; and hence it is advantageous to every organism that its several organs and structures should vary in their demands for nourishment from the general store. In this way the various parts fit in with each other, so that the material which is unsuited for one is used by another, and the greatest economy results.

Sir James Paget, years ago, enunciated this view in his surgical pathology, and showed that every part may be looked upon as an excretion to every other in highly complex organisms. Some organs perform this function to a very great extent. An example will render this phenomenon clear. A stag feeds upon grass, a large portion of which is dissolved by the process of digestion and passes into the circulating store of nutritive matter, the blood. Some of its constituents are oxidized for the production of energy; others repair the waste of the body in the adult animal, and build new tissues in the young one. Some of the remaining elements of the food are excreted from the body. But a large quantity of calcareous salts have been derived from the herbage on which the animal lives. This is drawn from the circulating fluid for the production of horns. The same salts that nourish the horns of

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HORNS OF RUMINANTS.

the male form the bones of the young in the female when this sex is hornless. All animals which feed on large quantities of comparatively slightly nutritious matter, with few exceptions, have a complicated digestive apparatus; and a large quantity of innutritious material is consequently dissolved and absorbed into the blood. In such animals there is a strong tendency to the production of largely developed skeletons or cutaneous organs. It is not difficult to see that the formation of such structures would probably be of great advantage to the individual, by relieving the special excretory organs by which they would otherwise be removed from the blood. Most insects which feed on material containing but little nutriment in comparison to its bulk produce cutaneous horns, knobs, and enormously developed mandibles. The thoracic appendages of the Lamellicorn beetles and the great jaws of the stag beetle may serve as examples, whilst the grotesque beaks of the Toucans and Hornbills may have had a similar origin.

The Ruminantia amongst the Mammalia are especially characterized by their strong tendency to produce horns, and by their extremely complex digestive apparatus, which enables them to dissolve an exceedingly large percentage of the saline constituents of their food. This is probably a necessary consequence of their peculiar mode of diges