Its main foundation is the nature of transmission, the fact that parents transmit their peculiarities to children, but that at the same time the children diverge from their parents in some respects and also differ from each other. It is for this reason that animals of the same kind are not all alike, but differ in all directions from the average type. Without this so-called variation it would be wholly impossible for one animal species to develop into another. All that is necessary for the formation of a new species is that the divergence from the central type become greater and that it goes on in the same direction until this divergence has become so great that the new animal no longer resembles the one from which it descended. But where is that force that could call forth the ever growing variation in the same direction? Lamarck declared that this was owing to the usage and much exercise of certain organs; that, owing to the continuous exercise of certain organs, these become ever more perfected. Just as the muscles of men’s legs get strong from running much, in the same way the lion acquired its powerful paws and the hare its speedy legs. In the same way the giraffes got their long necks because in order to reach the tree leaves, which they ate, their necks were stretched so that a short-necked animal developed to the long-necked giraffe. To many this explanation was incredible and it could not account for the fact that the frog should have such a green color which served him as a good protecting color. To solve the same question, Darwin turned to another line of experience. The animal breeder and the gardener are able to raise artificially new races and varieties. When a gardener wants to raise from a certain plant a variety having large blossoms, all he has to do is to kill before maturity all those plants having small blossoms and preserve those having large ones. If he repeats this for a few years in succession, the blossoms will be ever larger, because each new generation resembles its predecessor, and our gardener, having always picked out the largest of the large for the purpose of propagation, succeeds in raising a plant with very large blossoms. Through such action, done sometimes deliberately and sometimes accidentally, people have raised a great number of races of our domesticated animals which differ from their original form much more than the wild kinds differ from each other. If we should ask an animal-breeder to raise a long-necked animal from a short-necked one, it would not appear to him an impossibility. All he would have to do would be to choose those having partly longer necks, have them inter-bred, kill the young ones having narrow necks and again have the long-necked inter-breed. If he repeated this at every new generation the result would be that the neck would ever become longer and we would get an animal resembling the giraffe.