114 -4iV INTRODUCTION TO ZOOLOGY. 



■ cellular forms, through successive stages, to highly complex 

 organisms. 

 8. The causes ■which have determined this process of evolu- 



. tion are believed to be as follows : — Slight variations from 

 the parental type of any animal arise from time to time, some- 

 times fortuitously, but sometimes probably in consequence 

 of the influence of surrounding circumstances of soil, climate, 

 food, etc., which may be spoken of compendiously as the 

 environment of the animal. The large number of ofEspring 

 produced by each animal kind causes a keen struggle for 

 existence between them. Any variation that gives its 

 possessor some little advantage over the others will I'esult in 



, the survival of tixB fittest. The survivor will probably 

 transmit its peculiarity, according to the laws of heredity, 

 to its ofEspring, and thus an improved variety will be es- 

 tablished. By such small improvements, accumulating 

 through ages of time, the evolution of the animal kingdom 

 is understood to have been accomplished. The same pro- 

 cesses are still going on, and we may therefore expect to 

 learn much more about their operation. 



