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. 



