Tendency of Species to form Varieties. 



0293 



Three Papers on the Tendency of Species to form Varieties ; and on 

 the Perpetuation of Varieties and Species by Natural Means 

 of Selection. * 



1. Extract from an unpublished work on Species by C. Darwin, Esq., consisting of 

 a portion of a Chapter intituled, " On the Variation of Organic Beings in a 

 state of Nature; on the Natural Means of Selection; on the Comparison of 

 Domestic Kaces and true S]>ecies.'' 



DeCandolle, in an eloquent passage, has declared that all nature 

 is at war, one organism with another, or with external nature. 

 Seeing the contented face of nature, this maj' at first well be doubted ; 

 but reflection will inevitably prove it to be true. The war, however, 

 is not constant, but recurrent in a slight degree at short periods, and 

 more severely at occasional m.ore distant periods ; and hence its 

 effects are easil}^ overlooked. It is the doctrine of Malthus applied 

 in most cases with tenfold force. As in every climate there are sea- 

 sons, for each of its inhabitants, of greater and less abundance, so all 

 annually breed ; and the moral restraint which in some small degree 

 checks the increase of mankind is entirely lost. Even slow-breedincr 

 mankind has doubled in twenty-five years ; and if he could increase 

 his food with greater ease, he would double in less time. But for 

 animals without artificial means, the amount of food for each species 

 must, on an average, be constant, whereas the increase of all organisms 

 tends to be geometrical, and in a vast majority of cases at an enormous 

 ratio. Suppose in a certain spot there are eight pairs of birds, and 

 that only four pairs of them annually (including double hatches) rear 

 only four young, and that these go on rearing their young at the same 

 rate, then at the end of seven years (a short life, excluding violent 

 deaths, for any bird) there will be 2048 birds, instead of the original 

 sixteen. As this increase is quite impossible, we must conclude 

 either that birds do not rear nearly half their young, or that the 

 average life of a bird is, from accident, not nearly seven years. Both 

 checks probably concur. The same kind of calculation applied to all 

 plants and animals affords results more or less striking, but in very 

 few instances more striking than in man. 



Many practical illustrations of this rapid tendency to increase are 

 on record, among which, during peculiar seasons, are the extraordi- 

 nary numbers of certain animals ; for instance, during the years 1826 



* Reprinted frtnn the ' Proceedings of the Liunean Society.' 



