Darwin- Wallace Celebration. 107 



of the centrifugal governor of the steam engine, which checks 

 and corrects any irregularities almost before they become 

 evident ; and in like manner no unbalanced deficiency in the 

 animal kingdom can ever reach any conspicuous magnitude, 

 because it would make itself felt at the very first step, by 

 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 organized beings the many 

 lines of divergence from a central type, the increasing 

 efficiency and power of a particular organ through a suc- 

 cession 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 considerably in more essential characters. It also 

 furnishes us with a reason for that " more specialized 

 structure " which Professor 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. 



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 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 organized beings, their extinction and succession 

 in past ages, and all the extraordinary modifications of form, 

 instinct, and habits which they exhibit. 



Ternate, February, 1858. 



