92 Darwin- Wallace Celebration. 



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 in- 

 crease are on record, among which, during peculiar seasons, 

 are the extraordinary 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 

 introduced 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 necessarily stop as soon as the country 

 was fully stocked ; and yet we have every reason to believe, 

 from what is known of wild animals, that all would pair in 

 the spring. In the majority of cases it is most difficult to 

 imagine where the checks fall though generally, no doubt, 

 on the seeds, eggs, and young ; but when we remember how 

 impossible, even in mankind (so much better known than any 

 other animal), it is to infer from repeated casual observations 

 what the average duration of life is, or to discover the 

 different percentage of deaths to births in different countries, 

 we ought to feel no surprise at our being unable to discover 

 where the check falls in any animal or plant. It should 

 always be remembered, that in most cases the checks are 

 recurrent yearly in a small, regular degree, and in an extreme 

 degree during unusually cold, hot, dry, or wet years, according 

 to the constitution of the being in question. Lighten any 

 check in the least degree, and the geometrical powers of 

 increase in every organism will almost instantly increase the 

 average number of the favoured species. Ncature may be- 

 compared to a surface on which rest ten thousand sharp 



