420) THE CAUSES OF THE XI 
I don’t care how slight and trivial, so long as it is 
defined and constant, and does not depend on 
sexual peculiarities, then all naturalists agree in 
calling them two species; that is what is meant 
by the use of the word species—that is to say, it 
is, for the practical naturalist, a mere question of 
structural differences.! 
We have seen now—to repeat this point once 
more, and it is very essential that we should 
rightly understand it—we have seen that breeds, 
known to have been derived from a common stock 
by selection, may be as different in their structure 
_ from the original stock as species may be distinct 
from each other. 
But is the like true of the physiological charac- 
teristics of animals? Do the physiological differ- 
ences of varieties amount in degree to those 
observed between forms which naturalists call 
distinct species? This is a most important point 
for us to consider. 
As regards the great majority of physiological 
characteristics, there is no doubt that they are 
capable of being developed, increased, and modi- 
fied by selection. 
There is no doubt that breeds may be made as 
different as species in many physiological charac- 
ters. I have already pointed out to you very 
1 [| lay stress here on the practical signification of ‘‘ Species.” 
Whether a physiological test between species exist or not, it is 
hardly ever applicable by the practical naturalist. 
