Darwin and Descent 185 
belonging to the same genera or: families, have really 
started into life all at once, the fact would be fatal to the 
theory of descent with slow modification through natural 
selection ’’ (p. 302). 
Here “‘ the belief in the transmutation of species,” or 
descent with modification, is treated as synonymous with 
“the theory of descent with slow modification through 
natural selection ; ” but it has nowhere been explained that 
there are two widely different ‘“‘ theories of descent with 
slow modification through natural selection,” the one of 
which may be true enough for all practical purposes, while 
the other is seen to be absurd as soon as it is examined 
closely. The theory of descent with modification is not 
properly convertible with either of these two views, for 
descent with modification deals with the question whether 
species are transmutable or no, and dispute as to the 
respective merits of the two natural selections deals with 
the question how it comes to be transmuted ; nevertheless, 
the words ‘‘ the theory of descent with slow modification 
through the ordinary course of things’”’ (which is what 
“descent with modification through natural selection ” 
comes to) may be considered as expressing the facts with 
practical accuracy, if the ordinary course of nature is 
supposed to be that modification is mainly consequent on 
the discharge of some correlated function, and that modifi- 
cation, if favourable, will tend to accumulate so long as the 
given function continues important to the wellbeing of the 
organism ; the words, however, have no correspondence 
with reality if they are supposed to imply that variations 
which are mainly matters of pure chance and unconnected 
in any way with function will accumulate and result in 
specific difference, no matter how much each one of them 
may be preserved in the generation in which it appears. 
In the one case, therefore, the expression natural selection 
may be loosely used as a synonym for descent with modifi- 
cation, and in the other it may not. Unfortunately with Mr. 
