The Making of Species 
finttum.” From this “Law” it follows, says 
Romanes, on p. 13 of vol. tii. Darwin and after 
Darwin, that “no matter how infinitesimally 
small the difference may be between the average 
qualities of an isolated section of a species com- 
pared with the average qualities of the rest of 
that species, if the isolation continues sufficiently 
long, differentiation of specific type is necessarily 
bound to ensue.” 
This deduction involves two important assump- 
tions. The first is, that in each of the separated 
portions of the given species there is a constant 
cause of variation operating in one direction in 
the case of one portion and in another direction 
in the case of the other. This assumption is, 
unfortunately, not founded on fact. If we were 
to take one hundred race-horses and shut them 
up in one park and one hundred cart-horses and 
shut them up in another park, and prevent the 
interbreeding of the two stocks, we should, if 
Romanes’s tacit assumption be true, see the two 
types diverge more and more from one another. 
We know that as a matter of fact they will tend, 
generation after generation, to become more like 
one another. Galton’s Law of Regression, of 
which we have already spoken, and which is 
supported by ample evidence, clearly negatives 
this tacit assumption made by Romanes and 
Gulick. The second assumption upon which 
their reasoning is based is that there is no limit 
374 
