FOOT-NOTES TO EVOLUTION.

What is the cause of variety in life?

I.

THE KINSHIP OF LIFE.

No one with good eyes and brains behind them has ever looked forth on the varied life of the world, on forest or meadow or brook or sea, without at least once asking himself the question, "What is the cause of Nature's endless variety?" We see many kinds of birds and trees and insects and fishes and flowers and blades of grass, and yet when we look closely we find not one blade of grass in the meadow quite like another blade. The green cloak which covers the brown earth is the shield under which millions of organisms, brown or green, carry on their life work; yet not one organism in the world in body or mind is the exact measure of its neighbour. But with all this the real variety in life is far greater than that which appears.

Each kind of animal or plant, that is, each set of forms which in the vicissitudes of the ages has become segregated and set off from its neighbours, is called in biology a species. The number of these species is great beyond any ordinary conception. I have an old book in my library, the tenth edition of the Systema Naturæ,

What is a species ?

The number of species.

published by Linnæus in 1758. This book treats of all the species of animals known a century and a half ago. In its eight hundred and twenty-three pages some four thousand different kinds of animals are named and described. But for every one of these enumerated by Linnæus, more than two hundred kinds are known to the modern naturalist, and the number of species still unknown doubtless exceeds that of those already recorded. Every year since 1864 there has been published in London a plump octavo volume known as the Zoölogical Record. Each of these volumes, larger than the whole Systema Naturæ, contains the names of animals new to science added to our list during the year of which it treats. And in the record of each year we find the names of about three times as many animals as are mentioned in the Systema Naturæ. Yet the field shows no signs of exhaustion. As these volumes stand on the shelf together it is easy to see that the later volumes are the thickest, and that the record for the present year is the largest of all. Moreover, what is true of the increase of knowledge in systematic zoölogy is even more marked in the case of botany. Such, then, is the variety of life on the globe -a variety of which Linnæus and his successors had never dared to dream.

And yet, great as this variety is, there are, after all, only a few types of structure among all animals and plants, some three or four or eight or ten general modes of development, and all the rest are modifications from these few types. It is, moreover, true that all living

The unity of type.

forms are but series of modifications and extensions of one single plan of structure. All have the same framework of cells, and in each cell we find the same ultimate substance the mysterious semi-fluid network of proto

plasm, which is, so far as we know, the physical basis of all life; and the equally mysterious nuclear substance or chromatin which in some fashion presides over all the movements of the protoplasm and is the physical basis of the phenomena of heredity. The same laws of heredity, variability, and of response to outside stimulus hold in all parts of the organic world. All organisms have the same need of reproduction. All are forced to make concession after concession to their surroundings, and in these concessions all progress in life consists. And at last each organism or each alliance of organisms must come to the greatest concession of all, which we call death.

The unity in life is then not less a fact than is life's great diversity. Whatever the emphasis we may lay

Unity in variety. upon the diversity of life, the essential unity of all organisms must not be forgotten. This fundamental likeness among widely varied forms stands as the basis of all classification. It is this only which makes classification possible or conceivable. These bonds of union, which are real as distinguished from resemblances which are merely superficial or apparent, are known to the naturalist as homology. The existence of homologies is the fundamental fact in biological science. It has been regarded as a mystery of mysteries, but this mystery assumes the form of natural law in the light of the plain fact that identity of structure is the simple result of identity of parentage. Homology in any form is simply the stamp of heredity. In other words, homology means blood-relationship. The simplest explanation is the truest and would long ago have been recognized had it not been for prejudices of various sorts-theological prejudices that saw the image of God in man only, and scientific prejudices which arose from the surface study of surfaces. For it is the

inside of an animal which tells the real history of its ancestry; its outside tells us only where its ancestors have been.

The meaning of homology.

It is perfectly certain that homology represents some real law of Nature, something other than the results of mere chance. When I compare my arm with that of my neighbour, I find differences in size and proportions. But these are superficial, and there is the underlying correspondence of each bone and muscle, each nerve fibre, artery, and vein. When I compare my arm with the fore leg of a dog I find more striking differences, for the dog's station in life is quite unlike my own, and he uses his arm for different purposes. When I compare my arm with the wing of a bird or the pectoral fin of a fish, the results are still similar. Though the differences in each succeeding case become more and more striking, and the resemblance less easy to trace, yet the same resemblances exist, and a closer study shows that these resemblances far outweigh the differences.

We say, then, that homology is real, and whatever power or cause has acted on fishes to provide them with pectoral fins has given to birds wings, to the dog fore legs, and to me and my neighbour arms. The arms are appendages more specialized-that is, more highly finished and suited to more purposes than the others—but all are formed of the same pieces, arranged in the same way. When I compare my arm, however, with the claw. of a lobster, the limb of a tree, or the arm of a starfish, all resemblances in gross structure disappear, and we have only the analogies connected with similarity of function. The ultimate homology of cell for cell, however, remains even here with all that this may signify.

Now the problem before us is this: What is the

The origin of variety and the origin of homology.

The origin of life unknown.

origin of variety in life, and how does it come that this variety is based on essential unity? Or, in other words, what is the origin of species, and what is the origin of homology? Obviously, neither of these questions can be answered without considering the other, and obviously both presuppose the existence of life. As to the origin of life, we have as yet no basis for speculation. We can only say as a matter of fact that life exists on the earth, which was once lifeless. How the first organism came to be we can not even guess. By what clashing of elements the vital spark came forth, and whether like causes can or do still produce like effects, no one can say. The spontaneous generation of organisms has never been seen, nor with our dull senses and clumsy instruments could it ever be seen; for an organism without a history, untouched by heredity, unselected by struggle, unaffected by environment, a coin fresh from the mint of creation, would be a fragment of pristine simplicity as far beyond our grasp as the molecules of the chemist. It is likely that it is indeed a molecule, and a molecule in size compares with a drop of water much as an orange compares with the sun. If spontaneous generation exists, such creatures as bacilli and infusoria, small though they are, are not the products of it; for these little creatures have their life history, their habits, and their heredity as firmly fixed as those of the dog or the oak. A life history presupposes a long ancestry, and it is absurd to expect such battlescarred organisms as the least we know to spring full developed from the combination of any of the component atoms.

The origin of life is as yet beyond the reach of speculation. We can not even bring it under investigation,