134 Evolution and Adaptation 
some cases been of high importance to an early progenitor ; 
(3) the changed conditions of life may account for some of 
the useless organs; (4) reversion accounts for others; (5) the 
complex laws of growth account for still others, such as 
correlation, compensation of the pressure of one part on 
another, etc.; (6) the action of sexual selection is responsible 
for many characters not to be explained by natural selection. 
Admitting that there may be cases that can be accounted for 
on one or the other of these six possibilities, yet there can be 
no doubt that there are still a considerable number of specific 
characters that cannot be explained in any of these ways. 
I do not think that Darwin has, by any means met this 
objection, even if all these six possibilities be admitted as 
generally valid. 
Amongst the “ miscellaneous objections ” to his theory that 
Darwin considers we may select the most important cases. 
The following paragraph has been sometimes quoted by later 
writers to show that Darwin saw, to a certain extent, the 
insufficiency of fluctuating variations as a basis for selection. 
What he calls here “ spontaneous variability ” refers to sudden 
and extensive variations, or what we may call discontinuous 
variations. ‘In the earlier editions of this work I under- 
rated, as it now seems probable, the frequency and importance 
of modifications due to spontaneous variability. But it is 
impossible to attribute to this cause the innumerable struc- 
tures which are so well adapted to the habits of life of each 
species. I can no more believe in this, that the well-adapted 
form of a race-horse or greyhound, which before the principle 
of selection by man was well understood, excited so much 
surprise in the minds of the older naturalists, can thus be 
explained.” 
Darwin appears to mean by the latter part of this state- 
ment, that he cannot believe that such sudden and great 
variations as have caused a peach tree to produce nectarines 
can account for the wonderful adaptations of organisms; but 
