1 92 1.] Suhspecics and Evolution. 529 



has no intermediate form, as suitable intermediate terrain 

 does not exist. 



It also seems that distance in isolation is of no consequence 

 in permitting the union of two obvious geographical races 

 under one species. We must all admit interru})ted distri- 

 bution, and who is going to define the limit of such 

 interruption ? A large number of New World and Old 

 World forms have been united as geographical races of one 

 species, even when no intermediate forms either do or can 

 occur. If, then, we admit an interruption of 3000 miles, 

 why not admit an interruption of 6000 miles or even greater 

 distances ? 



If we were to accept the view^ that intermediate forms are 

 essential for a true geographical race, we could not concede 

 that a trinomial could be used except for purely environ- 

 mental differences among contiguous races : neither could 

 we allow that any isolated geographical race, showing 

 purely environmental differences, is entitled to trinomial 

 nomenclature. 



It therefore appears that intermediate forms cannot be 

 made an essential factor in determining a true geographical 

 race, and that interrupted distril)ution, however extended, 

 should not be a factor in rejecting an obvious geographical 

 race. 



2. Causes of geographical variation. 



Lowe and Praed, whilst admitting that most geographical 

 races are due to environment, consider that in some cases 

 they are due to mutation. They challenge the whole Dar- 

 winian theory, and all principles of gradual evolution. They 

 are not advocates of the " little by little " theory. 



Let us examine Uarwiu's theory. It is ably summarised 

 by Professor Punnett in his book on Mendelism. Darwin's 

 theory briefly was : — 



In any species of plant or animal the reproductive 

 capacity tends to outrun the available food supply, and 

 the resulting competition leads to an inevitable struggle 

 for existence. Of all the individuals born, only a 



