1921.] Subspecies 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 most all admit interrupted 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 distribution, 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 Darwin’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 
