Consider, then, what must happen to a new-born generation of living things of any species. Some of the individuals will be stronger or sturdier or better suited to succeed in life in some way than the rest, many individuals will be weaker or less suited. In particular single cases any sort of luck or accident may occur, but on the whole the better equipped individuals will live and grow up and reproduce themselves and the weaker will as a rule go under. The latter will be less able to get food, to fight their enemies and pull through. So that in each generation there is as it were a picking over of a species, a picking out of most of the weak or unsuitable and a preference for the strong and suitable. This process is called Natural Selection or the Survival of the Fittest.[8]
It follows, therefore, from the fact that living things grow and breed and die, that every species, so long as the conditions under which it lives remain the same, becomes more and more perfectly fitted to those conditions in every generation.
But now suppose those conditions change, then the sort of individual that used to succeed may now fail to succeed and a sort of individual that could not get on at all under the old conditions may now find its opportunity. These species will change, therefore, generation by generation; the old sort of individual that used to prosper and dominate will fail and die out and the new sort of individual will become the rule,—until the general character of the species changes.
Suppose, for example, there is some little furry whitey-brown animal living in a bitterly cold land which is usually under snow. Such individuals as have the thickest, whitest fur will be least hurt by the cold, less seen by their enemies, and less conspicuous as they seek their prey. The fur of this species will thicken and its whiteness increase with every generation, until there is no advantage in carrying any more fur.
Imagine now a change of climate that brings warmth into the land, sweeps away the snows, makes white creatures glaringly visible during the greater part of the year and thick fur an encumbrance. Then every individual with a touch of brown in its colouring and a thinner fur will find itself at an advantage, and very white and heavy fur will be a handicap. There will be a weeding out of the white in favour of the brown in each generation. If this change of climate come about too quickly, it may of course exterminate the species altogether; but if it come about gradually, the species, although it may have a hard time, may yet be able to change itself and adapt itself generation by generation. This change and adaptation is called the Modification of Species.
Perhaps this change of climate does not occur all over the lands inhabited by the species; maybe it occurs only on one side of some great arm of the sea or some great mountain range or such-like divide, and not on the other. A warm ocean current like the Gulf Stream may be deflected, and flow so as to warm one side of the barrier, leaving the other still cold. Then on the cold side this species will still be going on to its utmost possible furriness and whiteness and on the other side it will be modifying towards brownness and a thinner coat. At the same time there will probably be other changes going on; a difference in the paws perhaps, because one half of the species will be frequently scratching through snow for its food, while the other will be scampering over brown earth. Probably also the difference of climate will mean differences in the sort of food available, and that may produce differences in the teeth and the digestive organs. And there may be changes in the sweat and oil glands of the skin due to the changes in the fur, and these will affect the excretory organs and all the internal chemistry of the body. And so through all the structure of the creature. A time will come when the two separated varieties of this formerly single species will become so unlike each other as to be recognizably different species. Such a splitting up of a species in the course of generations into two or more species is called the Differentiation of Species.