accumulating on either side of the mean which is most advantageous to the species. When the environment changes, if certain variations on one side of the established mean tend to bring the individuals which manifest them into closer adaptation to the new environment, these individuals will survive in the struggle for life, and thus the average character of the species will be slightly altered. No two bears have just the same amount of hair, no two moths have just the same length of proboscis, no two antelopes are exactly matched in fleetness. Now if increasing cold renders a thicker covering useful to the bear, or if the lengthening of a flowercalyx, due to a slight change in soil or quantity of sunlight, renders a longer proboscis useful to the moth, or if the immigration of a carnivorous animal makes it necessary for antelopes often to run for their lives, then in each generation the thickest-coated bears, the longest-tongued moths, and the fleetest antelopes will survive. Every individual variation in the direction of a heavier coat, a longer sucker, or a structure better adapted for fleeing will give its owner the advantage in the incessant struggle for life, and these peculiarities will be oftenest inherited, while individuals which do not vary, or which vary in the wrong direction, will have to migrate or die.1

The student of natural history, who realizes, however dimly, the prodigious complexity of the relations of the various species of animals and plants to each other, will perceive that the amount of variation thus preserved and enhanced must in the course of long ages become enormous. If a grain of sand were each year added to an ant-heap, it would in course of time become as large as Chimborazo. But these changes, directly caused by natural selection, are greatly

1 It is thus one of the great merits of the theory of natural selection, that it accounts for the phenomena of extinction of species,-which formerly could only be accounted for by the gratuitous and utterly indefensible hypothesis of periodical catastrophes or cataclysms.

aided and emphasized by other changes indirectly produced by correlation of growth, and also by what is called the law of use and disuse. By correlation of growth, or internal equilibration, we mean the effect produced upon any part of the organism by change in a related or neighbouring part. Let us suppose that it becomes advantageous to some feline animal, like the ancestor of the lion, to have large and powerful jaws. Since no two of our leonines would have jaws of exactly the same size and strength, natural selection would preserve all the strong-jawed individuals, while the weakjawed individuals would succumb in the struggle for life. In the course of many generations our race of leonines would possess on the average much larger and stronger jaws than at the period at which we began to consider it. But greater weight of jaw entails increased exertion of the muscles which move the jaw, so that these muscles, receiving more and more blood, will become permanently increased in size and power. The portions of the skull into which the jawbones fit will likewise receive an extra strain, and will consequently increase in rate of nutrition and grow to a larger size, so that the shape of the whole head will be altered. This increased weight of the head, and the increasingly violent activity of the muscles which move the jaws, entails a greater strain upon the vertebræ which support the head, and upon the cervical muscles which move it from side to side. The heightened nutrition of these bones and muscles will add to their weight, so that the shoulders and chest will be affected. There will be a tightening of the tendons, and probably a perceptible alteration in the relative lengths of the different bones and muscles throughout the anterior part of the body; and these changes, altering the animal's centre of gravity, will inevitably cause other compensating changes in the rest of the body. The legs, shoulders and haunches will be modified. Alterations in the weights bearing upon the chest will affect the growth of the lungs and the aeration

of the blood. And the stomach, intestines, and various secreting glands will respond to the requirements of all these nutritive changes. While, lastly, such deep-seated variations cannot fail to influence the nervous system of the animal, and to modify somewhat its temperament and its modes of life.

To illustrate the effects of use and disuse, let us reconsider the antelopes, of whom natural selection has so long preserved the swiftest and most quickly frightened individuals that they now rank among the fleetest and most timid of maminals. If all the lions and other swift carnivora of Africa were to become extinct, so that antelopes would no longer have to run for their lives, the slower and less easily alarmed individuals would begin to be preserved in as great numbers as the swifter and more timid ones, so that by and by the average speed and timidity of the race would be diminished. In all this we see merely the effects wrought by unaided natural selection. But it is a fundamental law of biology that functions are maintained at their maximum only through constant exercise. Freed from savage enemies, our antelopes would less frequently use the muscles concerned in running, and would less often exercise the mental faculties concerned in the rapid perception of approaching danger. Inevitably, therefore, they would, after several generations, diminish in speed, and become less alert and less timid. Here we see the effects of what is called the law of use and disuse. But to these we should also have to add the effects of correlation of growth. Decrease in speed, involving decrease in muscular tonicity, and rendering possible the assimilation of less concentrated food, would seriously modify the nutrition of the entire organism. The digestive tract would probably be enlarged, and larger and lazier bodies could not fail to be produced, both by the direct influence of the nutritive processes, and because natural selection would no longer necessitate the slaughter of all clumsy-bodied individuals. Thus in course of time the breed of antelopes

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would become so thoroughly altered as to constitute a distinct species from their graceful, swift, and timid ancestors. It is in just these ways that New Zealand birds, freed by insular isolation from the attacks of mammalian enemies, have grown large and clumsy, and have lost the power of flight which their partly-aborted wings show that they once possessed.

By the same kind of illustration we may form a rough notion of the way in which a single species bifurcates into two well-defined species. Suppose a race of ruminants to have been living in Africa before the introduction of carnivora, and suppose that, for sundry reasons, the vitality of the race was but little affected by moderate variations in the sizes of its individuals, so that while some were comparatively light and nimble, others were comparatively large and clumsy. Now introducing upon the scene the common ancestor of the lion and the leopard-by immigration either from Asia or from some other adjacent territory now submerged-let us note some probable features of the complex result. First, as regards the attacked ruminants, it is likely that in course of time the lightest and swiftest individuals, habitually taking refuge in flight, would have greatly increased both in fleetness and in timidity; the largest and most clumsy of the species, unable to save themselves by fleeing, would often be forced to stand and fight for their lives, and would thus ultimately have gained in size, strength, and courage; while those who were neither nimble enough to get out of the way nor strong enough to fight successfully would have all been killed off. And thus, after a while, by perpetual destruction of the means and preservation of the extremes, we should get two kinds of ruminant as different from one another as the antelope which escapes by his fleetness and cautious timidity, and the buffalo which boldly withstands the lion and not unfrequently conquers or repulses him. Secondly, let us observe what must have been going on all the while with the attacking carnivora. The lighter and less

powerful of these would find manifest advantage in crouching amid dense foliage and springing down upon unwary victims passing below. The larger and more powerful individuals would more frequently roam about the open country, attacking the larger ruminants and giving chase to the nimbler ones, and would thus increase in strength and fleetness. And thus there would be initiated such differences of size and habit as characterize the leopard and the lion.

It must be borne in mind that this is a purely hypothetical illustration, which does not pretend to give a complete account of the complex process. I have no idea that the differentiation between antelopes and buffaloes, or between lions and leopards, was accomplished in any such straightforward way as this. But while unduly simplifying the case, the illustration is undoubtedly sound in principle. No doubt the lion is so strong and so swift because only the strongest and swiftest lions have been able to prey at once upon buffaloes and upon antelopes. No doubt the antelope is so swift and so timid because only the swiftest and most quickly-frightened antelopes have been enabled to get away from the lion, and to propagate their kind. And no doubt in the process above described, we get a partial glimpse of some of the essential incidents in the past careers of these races.

All the foregoing illustrations unite in enforcing the conclusion that the direct and indirect effects of natural selection are by no means limited to slight or superficial changes in organisms. The student of physiology well knows that no change, however seemingly trivial, which ensures the survival of the organism in its fierce struggle for existence, can fail in the long run to entail so many other changes as to modify, more or less perceptibly, the entire structure. Even such a slight change as an increased thickness of the woolly coat of a mammal may, by altering the excretory power of the skin, affect the functions of the lungs, liver, and kidneys, and thus indirectly increase or diminish the size of the