40 THE PROBLEMS OF EVOLUTION 



for the convenience of classification, but phyloge- 

 netically the species includes not only the individ- 

 uals extant at a given time, but all of the succes- 

 sion of generations occurring before it loses its 

 identity in the production of other species. In 

 this succession there can be no sharp boundaries. 

 We are dealing with entities which may be sharply 

 limited in space but are inevitably diffuse in time. 

 Genetics has taught us that the mechanism of 

 heredity guarantees a maximum diversity of indi- 

 viduals constituting a species. The assemblage of 

 unit characters for which determiners are carried 

 by the chromosome complex of the species includes 

 many different expressions of the same fundamen- 

 tal character. Illustrations are common enough. 

 Hair color in man may be black, brown, red, or 

 blonde; skin varies in pigmentation from African 

 black to Caucasian fairness; eyes may be anything 

 from deep brown to pale blue. In the normal course 

 of sexual reproduction the various characters are 

 redistributed in such a thorough way that the 

 chance of two individuals receiving identical herit- 

 ages is negligible. Twins of the identical or mono- 

 chorial type apparently do so because they are 

 developed from equipotential portions of a single 

 developing zygote, separated at an early stage in 

 embryonic life. Their approximate identity em- 

 phasizes the more common differences of individ- 

 ual heritage. 



