Page:The Atlantic Monthly, Volume 18.djvu/428

420 the scrutiny of man. It is so far without the scope of his experience, his speculation even, that it is futile to attempt to surmise it; although certain interesting phenomena have attended the experiments of naturalists, especially those of Professor Jeffries Wyman. Darwin takes the subject up at the appearance of animal life, and seeks to work out the causes of the present variation among animals, and to detect the modus operandi by which the law of evolution has produced the multiform changes now apparent. "Natural Selection" is his password into the great workshop of Nature. The three great agencies at work there are the tendency of all animals to transmit their peculiarities to their offspring, the tendency of all animals to vary from their ancestors under varying influences around them, and the constant changes taking place in their surroundings. "Natural Selection" are the words chosen to express the action of animals under these conflicting forces. The power of external influence upon structure is exhibited in the remarkable results of man's treatment of plants and animals. The varieties of pears and apples are of his producing; and the different breeds of domestic animals are the direct consequences of his influence. The most astonishing instance of his power is shown in pigeons, which are made, by skilful breeders, to assume a great variety of shapes, colors, and habits. To what is this change due? If a rock-pigeon were left to itself, it would produce only rock-pigeons, unless some new influences were brought to bear by natural causes. Man gives a rock-pigeon some peculiar food, subjects it to some peculiar treatment, and the creature begins at once to change; that is, to accommodate itself to the new circumstances by which it is surrounded. The plastic nature undergoes an alteration correspondent to the new state of existence. In a few generations these varieties are indefinitely multiplied; and then, by crossing these varieties, new ones are again produced. What is then the state of affairs? We have a series of birds which, were they discovered in new countries, would be considered to be new species. They differ in shape and color, in the number of their vertebræ and ribs, the number of wing and tail feathers, the number of scales on their toes, and various other anatomical peculiarities. Here then is proof that animals, when exposed to influences different from those which surrounded their ancestors, take on new forms and characteristics. This is man's work. But is not man one of the many agents in the work of the Great Prime Mover? Let us suppose that the peculiar circumstances which produced a pouter or a fan-tail were to remain in force for centuries; would not pouters or fan-tails continue to procreate, and thus a new species be added to the genus? What, then, becomes of special creation of every species of animal, if man in a few years can produce a dozen forms out of one, any one of which dozen is so distinct as to be deemed a new species, were it Nature's work and not man's? The fact is thus demonstrated that animals become varied in accordance with variations in their surroundings. This simple fact, once substantiated, is the key to the whole subject. Man's influence ceases when he leaves the animal to itself. But Nature never leaves the animal to itself. Her changes are slight and slow; but they never go back. They are permanent, only to be reaffected as the environment alters. When we consider the incalculable, inconceivable lapse of time through which organic life has been swayed by the never-ceasing action of the forces around it, we can imagine what a vast variety of animal forms may have been evolved from some one primal ancestor.

Darwin endeavors to explain, in detail, how this differentiation takes place. The largest or strongest get the best food or the most attractive females, and then transmit their strength or their peculiarities to their progeny. These peculiarities are the results of