386 OUTLINES OF EVOLUTIONARY BIOLOGY 



Professor Asa Gray. A few quotations will suffice to indicate 

 his views : 



" De Candolle, in an eloquent passage, has declared that all 

 nature is at war, one organism with another, or with external 

 nature. ... It is the doctrine of Malthus applied in most 

 cases with tenfold force. . . . 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, 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." 



" 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. . . . 

 Finally, 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 

 individual 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 organisms, 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 

 inhabitants be small, as on an island, and free access to it from 

 other countries be circumscribed, and let the change of conditions 

 continue progressing (forming new stations), in such a case the 

 original inhabitants must cease to be as perfectly adapted to the 



