6308 Tendency of Species to form Varieties, 



accompanying weak feet, or great velocity making up for the absence 

 of defensive weapons ; for it has been shown that all varieties in which 

 an unbalanced deficiency occurred could not long continue their ex- 

 istence. The action of this principle is exactly like that 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 w^ould 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 diver- 

 gence from a central type, the increasing efficiency 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 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 pro- 

 gressive 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 progression 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. 



A. R. Wallace. 



Teinate, February, 1858. 



