148 



ANIMAL BIOLOGY 



FIG. 281. CHAMELEON OF SOUTHERN EUROPE. 



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 dis- 

 appearance of geo- 

 graphical 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 bene- 

 ficial variations live longer and leave more offspring, and because 

 of heredity transmit the desirable qualities to 

 some of their young. Variations which are dis- 

 advantageous 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 observa- 

 tions 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 environ- 

 ment. "Survival of the fittest" results from this 

 natural selection, but the selection occurs be- 

 tween animals of marked, not infinitesimal, dif- 

 ferences, 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. 



FIG. 282. EM- 

 BRYO OF A 

 TURTLE, show- 

 ing four gill slits^ 

 (Challenger Re- 

 port.) 



