130 ANIMAL PHYSIOLOGY. 
Vines has stated that the equivalent of parthenogenesis 
takes place in the male cell in plants.. Though this may be an 
objection to the universality of application of Brooks’s theory, 
it does not seem to us to be fatal to it as a whole. 
As has been pointed out,in a previous chapter, Darwin held 
that the differences that caused ultimately the formation of 
new groups of living forms were the result of extremely slow 
accumulation of variations, at first very minute. He every- 
where insists upon this. But, unquestionably, it is just here 
that the greatest difficulty is to be encountered in the Darwin- 
ian account of evolution. The chances agiainst the loss of the 
variation by breeding with forms that did not possess it seem 
to be numerous, hence various theories have been proposed to 
lessen the difficulty. 
Mivart introduced the doctrine of extraordinary births, be- 
lieving that variations were often sudden and pronounced. 
That they were so occasionally Darwin himself admitted; but 
he considered a theory like that of Mivart as a surrender, a 
resort to an explanation that verged in its character on the 
introduction of the supernatural itself. 
A view that has attracted much attention and caused a 
great deal of controversy, is that of Romanes, which was intro- 
duced in part to meet the difficulty just referred to; and to lessen 
the further one arising from the infertility of species with one 
another, as compared with the perfect fertility of varieties. It 
has often been noticed that, though the difference anatomically 
between varieties might be greater than between species, the 
above law as to fertility still held. Such a fact calls for ex- 
planation; hence Romanes has proposed his theory of “ physi- 
ological selection” (segregation, isolation). If it be admitted 
‘that some change may take place in the sexual organs of two 
forms so that the members of one are fertile with each other 
while those of the other are not, it will at once appear that 
they are as much isolated physiologically as if separated by an 
ocean. That such does take place is an assumption based on 
the great tendency in the reproductive organs to change; and 
it is claimed that, if this assumption be granted, that the main 
difficulty of Darwin’s theory will be removed, for the “ swamp- 
ing” action of intercrossing forms that vary slightly, or one of 
them not at all,in the given direction, will not occur. Romanes 
believes that forms that vary are fertile inter se, but not with 
the parent forms, which would meet the case fairly well. Cer- 
tain it is that species are not generally fertile with one an- 
