6294 



Tendency of Species 



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) ordi- 

 narily 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 ani- 

 mals, 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. 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, &c., should be well considered. Reflect on the enormous 

 multiplying power inherent and anmially in action in all animals ; 

 reflect on the countless seeds scattered by a hundred ingenious con- 

 trivances, year after year, over the whole face of the land ; and yet we 

 have every reason to suppose that the average percentage of each of 

 the inhabitants of a country usually remains constant. Finally, let it 

 be borne in mind that this average number of individuals (the exter- 

 nal conditions remaining the same) in each country is kept up by 



