All this impressed on Darwin’s mind the importance of the word “selection,” which appears in the title of his theory and the subtitle of his epoch-making book. Could it be possible that nature contained some principle or combination of principles, which performed among wild animals a part analogous to that of the breeder, among domestic animals? Darwin discovered that this is precisely what takes place. His famous theory may be formulated under the three following heads:
(3) The struggle for existence, with its resultant, survival of the fittest.
Darwin requires very little of heredity, and what he does ask is beyond dispute. It is enough for his theory if like begets like and “figs do not grow on thistles.”
Similarly with variation, the demands of his hypothesis are very slight. If it be conceded that variation is a fact, that offspring do vary from their parents and each other, it is enough. And who will dispute this in a world where no two creatures are exactly and in all particulars alike? The apparent contradiction that, heredity demands likeness, while variation requires difference, is confined to the surface--it is not real. The likeness is general while the difference is particular. A sheep may be born with shorter or longer legs, by variation; but it will be a sheep and not a horse, by heredity.
As an example of the working of the theory let us take Lamarck’s piece de resistance, the giraffe. Lamarck says: “We know that this animal, the tallest of mammals, inhabits the interior of Africa, and that it lives in localities where the earth, almost always arid and destitute of herbage, obliges it to browse on the foliage of trees and to make continual efforts to reach it. It has resulted from this habit, maintained for a long period in all the individuals of its race, that its forelegs have become longer than the hinder ones, and that its neck is so elongated that the giraffe, without standing on its hind legs, raises its head and reaches six meters in height (almost twenty feet).”
Lamarck thought this length of neck was acquired by “continual efforts to reach,” or, as Alfred Russell Wallace puts it in his criticism of Lamarck--“stretching.” Many critics ventilated their wit on this theory of Lamarck’s, under the impression that they were lampooning Darwin’s idea.