and natural selection, and the evolution which results from the former therefore furnishes experimental (i.e. conclusive) proof of the evolution which results from the latter.
The reduction towards the specific mean which interbreeding tends to bring about in the qualities of individuals as exhibited in their descendants, has caused some biologists to insist that isolation is a necessary antecedent to evolution.[1] But if we bear in mind the fact (to be more fully discussed in a future page) that evolution proceeds not on lines of traits, however favourable, which occur infrequently or abnormally, but on lines of traits common to all the individuals of the whole species, that is, in which every individual rises above or falls below the specific average; and bear in mind also, that generally those individuals who on the whole vary fortunately as regards these traits, survive and have offspring, whereas generally those who on the whole vary unfortunately are eliminated and have no offspring, we shall have no difficulty in understanding that evolution is quite possible in the absence of isolation. What is not possible in the absence of isolation is evolution on diverging lines. For instance, if the whole of a species of antelope inhabited an open plain under conditions common to all there might be evolution, but it would be in a direction common to the whole species. But if, owing to any circumstance, such as a deficiency of food supply, a portion of the species separated from the rest, and migrated permanently to forest land, then the two divisions of the species, being unable to interbreed,
- ↑ "Without isolation, or the prevention of free intercrossing, organic evolution is in no case possible. Isolation has been the universal condition of modification. Heredity and variability being given, the whole theory of organic evolution becomes a theory of the causes and conditions which lead to isolation "—Romanes.
