TENDENCY OF SPECIES TO FORM VAEIETIES. 47 



Seeing the contented face of nature, this may 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 more distant 

 periods; and hence its effects are easily overlooked. It is the 

 doctrine of Malthus applied in most cases with tenfold force. As 

 in every climate there are seasons, 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 man- 

 kind is entirely lost. Even slow-breeding 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, exclu- 

 ding 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 ex- 

 traordinary numbers of certain animals ; for instance, during the 

 years 1826 to 1828, in La Plata, when from drought some millions 

 of cattle perished, the whole country actually swarmed with mice. 

 Now I think it cannot be doubted that during the breeding-season 

 all the mice (with the exception of a few males or females in 

 excess) ordinarily pair, and therefore that this astounding increase 

 during three years must be attributed to a greater number than 

 usual surviving the first year, and then breeding, and so on till 

 the third year, when their numbers were brought down to their 

 usual limits on the return of wet weather. Where man has intro- 

 duced plants and animals into a new and favourable country, 

 there are many accounts in how surprisingly few years the whole 

 country has become stocked with them. This increase would 



