48 MESSllS. C. DARWIN AND A. WALLACE ON THE 



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 obser- 

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

 different percentage of deaths to births in diiferent 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, &c., should 

 be well considered. Reflect on the enormous multiplying power 

 inherent and annually in action in all animals ; reflect on the 

 countless seeds scattered by a hundred ingenious contrivances, 

 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, rinally, 

 let it be borne in mind that this average number of individuals 

 (the external conditions remaining the same) in each country is 

 kept up by recurrent struggles against other species or against 

 external nature (as on the borders of the Arctic regions, where 

 the cold checks life), and that ordinarily each indi\ddual of every 

 species holds its place, either by its own struggle and capacity of 

 acquiring nourishment in some period of its life, from the egg 

 upwards ; or by the struggle of its parents (in short-lived organ- 

 isms, when the main check occurs at longer intervals) with other 

 individuals of the same or different species. 



But let the external conditions of a country alter. If in a 

 small degree, the relative proportions of the inhabitants will in 

 most cases simply be slightly changed ; but let the number of 



