63 ON THE TENDENCY OF SPECIES TO FOEM VABIETIES. 



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 cor- 

 rects 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 difiicult 

 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 

 succession of allied species, and the remarkable persistence of 

 unimportant parts such as colour, texture of plumage and hairj 

 form of horns or crests, through a series of species differing con- 

 siderably 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 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 con- 

 ditions, 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. 



