8 POPULAR SCIENCE MONTHLY. 



of seven years (a short life, excluding violent deaths, for any bird) 

 there will be 2048 birds, instead of the original sixteen. As this in- 

 crease 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 re- 

 sults more or less striking, but in ver}^ 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 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 ex- 

 ception of a few males or females in excess) ordinarily pair, and there- 

 fore that this astounding increase during three years must be attributed 

 to a greater number than usual surviving the first year, and then breed- 

 ing, 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 re- 

 membered, 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. Nature may be compared to a surface 

 on which rest ten thousand sharp wedges touching each other and 

 driven inwards by incessant blows. Fully to realize these views much 

 reflection is requisite. Malthus on man should be studied; and all 

 such cases as those of the mice in La Plata, of the cattle and horses 

 when first turned out in South America, of the birds by our calculation, 

 etc., should be well considered. Reflect on the enormous multiplying 



