Page:First course in biology (IA firstcourseinbio00bailrich).pdf/386

 progress or evolution possible. The climate of the world is slowly changing, and animals must change to adapt themselves to it. A more sudden change of environment (surroundings) of animals occurs because of migration or isolation; these in turn are caused by the crowding of other animals or by the formation or disappearance of geographical barriers, such as deserts, water, mountain chains.

The young vary in many ways from their parents. Some have a more protective color or form, sharper claws, swifter movements, etc. The individuals possessing such beneficial variations live longer and leave more offspring, and because of heredity transmit the desirable qualities to some of their young. Variations which are disadvantageous for getting food, defense, etc., cause shorter life and fewer offspring. Thus the fittest survive, the unfit perish; an automatic natural selection occurs.

Darwin taught that variations are infinitesimal and gradual. Recent experiments and observations seem to show that many variations are by sudden jumps, somewhat resembling so-called "freaks of nature." As to whether these "sports," or individuals with new peculiarities, survive, depends upon their fitness for their environment. "Survival of the fittest" results from this natural selection, but the selection occurs between animals of marked, not infinitesimal, differences, as Darwin taught. Darwin's theory is probably true for species in the usual state of nature; the new theory (of De Vries) is probably true for animals and plants under domestication and during rapid geographical changes.

, showing four gill slits. (Challenger Report.)