THE INTEGRATION OP THE ORGANIC WORLD. 405 



late date. So, too, of much commensalism and many 

 mutually-beneficial associations. The reciprocal services of 

 ants and aphides must have originated since the Hymenoptera 

 and Hemiptera became established types, and since the days 

 when certain insects of the ant-type had become social, and 

 since the days when aphides had become degraded members 

 of their order : both dates being relatively recent. And still 

 more recent must have been the commensalism between the 

 ants and the many species of other insects which inhabit 

 their nests. 



Leaving out relations of the kinds just named, it seems 

 that down from those between carnivores and their prey to 

 those between lice and their hosts, such relations profit one 

 of the two species concerned and injure the other, and that 

 there the matter ends. But it does not end there; for that 

 multiplication of effects to which people are usually blind, 

 brings about changes which, as hinted above, though inju- 

 rious to the individual are beneficial to the species, and 

 which, when not beneficial to the species, are often beneficial 

 to the aggregate of species. 



Even where animals of one class live by devouring animals 

 of another class, we see, on looking beyond the immediate 

 results, certain remote results that are advantageous. In 

 the first place the process is one by which inferior individuals 

 the least agile, swift, strong, or sagacious are picked out 

 and prevented from leaving posterity and lowering the 

 average quality of their kind. At the same time individuals 

 made feeble by injury or old age, are among those to be 

 killed and saved from suffering prolonged pains : the evils of 

 death by disease and starvation being thus limited to the pre- 

 datory animals, relatively small in their numbers. Mean- 

 while a check is put on undue multiplication. Where a tract 

 of country has been overrun by rabbits, weasels, thriving on 

 the abundant supply of food, presently become numerous 

 enough to bring the population of rabbits within moderate 

 limits; and by doing this benefit not only all those kinds of 



