Chap. V. 
SUMMAKY. 
169 
for in a district where many species of any genus are 
found—that is, where there has been much former 
variation and differentiation, or where the manufactory 
of new specific forms has been actively at work—there, 
on an average, we now find most varieties or incipient 
species. Secondary sexual characters are highly vari¬ 
able, and such characters differ much in the species 
of the same group. Variability in the same parts of 
the organisation has generally been taken advantage 
of in giving secondary sexual differences to the sexes 
of the same species, and specific differences to the 
several species of the same genus. Any part or organ 
developed to an extraordinary size or in an extraor¬ 
dinary manner, in comparison with the same part or 
organ in the allied species, must have gone through an 
extraordinary amount of modification since the genus 
arose ; and thus we can understand why it should often 
still be variable in a much higher degree than other 
parts; for variation is a long-continued and slow pro¬ 
cess, and natural selection will in such cases not as 
yet have had time to overcome the tendency to further 
variability and to reversion to a less modified state. But 
when a species with any extraordinarily-developed organ 
has become the parent of many modified descendants 
—which on my view must be a very slow process, 
requiring a long lapse of time—in this case, natural 
selection may readily have succeeded in giving a fixed 
character to the organ, in however extraordinary a 
manner it may be developed. Species inheriting nearly 
the same constitution from a common parent and ex¬ 
posed to similar influences will naturally tend to present 
analogous variations, and these same species may occa¬ 
sionally revert to some of the characters of their ancient 
progenitors. Although new and important modifica¬ 
tions may not arise from reversion and analogous varia- 
1 
