98 Darwin- Wallace Celebration. 



This sketch is most imperfect ; but in so short a space I 

 cannot make it better. Your imagination must fill up very 

 wide blanks. 



C. DARWIN. 



III. On the Tendency of Varieties to depart indefinitely from 

 the Original Type. By ALFRED RUSSEL WALLACE. 



One o the strongest arguments which have been adduced 

 to prove the original and permanent distinctness o species is, 

 that varieties produced in a state of domesticity are more or 

 less unstable, and often have a tendency, if left to themselves, 

 to return to the normal form of the parent species ; and this 

 instability is considered to be a distinctive peculiarity of all 

 varieties, even of those occurring among wild animals in a 

 state of nature, and to constitute a provision for preserving 

 unchanged the originally created distinct species. 



In the absence or scarcity of facts and observations as to 

 varieties occurring among wild animals, this argument has 

 had great weight with naturalists, and has led to a very 

 general and somewhat prejudiced belief in the stability of 

 species. Equally general, however, is the belief in what are 

 called " permanent or true varieties," races of animals 

 which continually propagate their like, but which differ so 

 slightly (although constantly) from some other race, that the 

 one is considered to be a variety of the other. Which is the 

 variety and which the original species, there is generally no 

 means of determining, except in those rare cases in which 

 the one race has been known to produce an offspring unlike 

 itself and resembling the other. This, however, would seem 

 quite incompatible with the "permanent invariability of 

 species," but the difficulty is overcome by assuming that 

 such varieties have strict limits, and can never again vary 

 further from the original type, although they may return to 

 it, which, from the analogy of the domesticated animals, is 

 considered to be highly probable, if not certainly proved. 



It will be observed that this argument rests entirely on the 

 assumption, that varieties occurring in a state of nature are 

 in all respects analogous to or even identical with those of 

 domestic animals, and are governed by the same laws as 

 regards their permanence or further variation. But it is the 

 object of the present paper to show that this assumption is 

 altogether false, that there is a general principle in nature 

 which will cause many varieties to survive the parent species, 



