II ON THE TENDENCY OF VARIETIES, ETC. 33 
rendering existence difficult and extinction almost sure soon 
to follow. An origin such as is here advocated will also 
agree with the peculiar character of the modifications of form 
and structure which obtain in organised beings—the many 
lines of divergence from a central type, the increasing effici- 
ency and power of a particular organ through a succession of 
allied species, and the remarkable persistence of unimportant 
parts, such as colour, texture of plumage and hair, form of 
horns or crests, through a series of species differing consider- 
ably in more essential characters. It also furnishes us with 
a reason for that “more specialised structure” which Pro- 
fessor Owen states to be a characteristic of recent compared 
with extinct forms, and which would evidently be the result 
of the progressive modification of any organ applied to a 
special purpose in the animal economy. 
Conclusion 
We believe we have now shown that there is a tendency 
in nature to the continued progression of certain classes of 
varieties further and further from the original type—a pro- 
gression to which there appears no reason to assign any 
definite limits—and that the same principle which produces 
this result in a state of nature will also explain why domestic 
varieties have a tendency, when they become wild, to revert 
to the original type. This progression, by minute steps, in 
various directions, but always checked and balanced by the 
necessary conditions, subject to which alone existence can be 
preserved, may, it is believed, be followed out so as to 
agree with all the phenomena presented by organised 
beings, their extinction and succession in past ages, and all 
the extraordinary modifications of form, instinct, and habits 
which they exhibit. 
