THE ORIGIN AND HISTORY OF ANIMAL LIFE 



625 



Bar eye 



A red-eyed 

 normal female 



Vestigial wing 



Figure 440. Mutations in the fruit fly, Dwsophila, referred to by someone as "God's gift to 

 geneticists." These are only a few of the thousands of mutations that are known to have oc- 

 curred in this httle fly. 



nization with subsequent isolation appears to 

 account for this situation. The original snail 

 or snails in each valley possessed a set of 

 genes that differed from those of snails in 

 other valleys, since the genetic constitution 

 of no two individuals (except identical 

 twins) is alike. This fact, plus the probable 

 mutations that took place and were per- 

 petuated, would account for the diversity 

 noted. 



The tvpes of isolation cited may be called 

 geographic isolation. Other types of isola- 

 tion also occur; for example, seasonal isola- 

 tion exists when the breeding season of two 

 groups of animals occurs at different sea- 

 sons of the year; sexual isolation occurs when 

 copulation is prevented for physiologic or 

 psychologic reasons, when copulation is 

 impossible on account of the morphology of 

 the reproductive organs, when the sperma- 

 tozoa are unable to reach or unite with the 

 eggs, when the young hybrids that may de- 

 velop from the fertilized eggs fail to reach 



sexual maturity, or when sexually mature 

 hybrids are unable to reproduce. Examples of 

 all types of isolation mentioned might be 

 cited and are more numerous than one 

 would suppose. 



THE BACKGROUND 

 OF MAN 



Zoologists do not maintain that human 

 beings have descended from monkeys, as 

 many people believe; but from a study of 

 living primates and of their fossil remains, 

 they are of the opinion that man and 

 monkey both had common ancestors that 

 lived many thousands of years ago. One au- 

 thority's idea of primate descent is that the 

 common monkeys of the present day sepa- 

 rated from the ancestral stock at a very 

 eady period. The ancestral anthropoids gave 

 rise to the anthropoid apes that are living 

 today, such as the chimpanzee and gorilla 



