148 



BEGINNERS' ZOOLOGY 



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 oc- 

 curs 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 



FIG. 281. -CHAMELEON OF SOUTHERN EUROPE. colour or fornlj 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, defence, 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- fic 282. — Em- 

 ment. " Survival of the fittest " results from this bryo of a 



natural selection, but the selection occurs be- Turtle, show- 

 ing four gill slits, 

 tween animals of marked, not infinitesimal, dif- (Challenger Re- 

 ferences, as Darwin taught. Darwin's theory is port.) 

 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. 



