1/8 EVOLUTION OF TO-DAY. 



have recognized nine. But the important fact is 

 not as to the number or exact limits of these divi- 

 sions, but that they do exist ; that it is everywhere 

 recognized that organisms are grouped together in 

 masses, which are separated from each other by bar- 

 riers. The details are a secondary feature. 



The question of course arises as to what is the 

 cause of this bond of unity found in the inhabitants 

 of the separate zoological regions. Doubtless, some 

 of it may be due to like conditions affecting all 

 alike ; but this is not sufficient to explain all. The 

 South American monkeys all have six molar teeth, 

 while none of the Old- World monkeys have more 

 than five ; and surely unlike conditions can never 

 account for this sort of difference. It was long ago 

 pointed out by Darwin that the theory of genetic 

 descent offers an explanation for such facts. Sepa- 

 rated as these regions are from each other, the 

 species of any comparatively small, recent group 

 will all be somewhat close relations, and the com- 

 mon bond of union is that of inheritance. The first 

 monkeys reaching South America from the north, 

 for example, had six molar teeth, and the present 

 species, which are their descendants, though in 

 many features they have become distinct, have re- 

 tained this feature of likeness ; and since now the 

 physical barriers prevent the immigration of other 

 forms, it results that all of the South American 

 monkeys show a fundamental likeness. Barriers 

 will thus tend to confine within their limits, species 

 somewhat closely related to each other. Australia 

 contains nearly all of the marsupials of the world ; 



