406 PHYSIOLOGICAL DEVELOPMENT. 



plants which are being eaten down, and all those other ani- 

 mals which live on such plants, but also the rabbits them- 

 selves; since, increasing beyond the means of subsistence, a 

 large part of them would, if not killed, die of hunger. Be- 

 tween aphides and lady-birds we see a connexion of like 

 nature: great increase of the first yielding abundant food to 

 larva? of the second, ending after a season or so in swarms of 

 lady-birds, and consequently of their larvae, whereby the 

 aphides, immensely diminished, cease so greatly to injure 

 various plants and the animals dependent on them. Even 

 minute parasites, by the evils they inflict on one species, 

 profit others : instance the enormous destruction of flies 

 which a microscopic fungus caused a few years ago a 

 destruction which relieved not only man but all the animals 

 which flies irritate: often so much as to hinder them from 

 feeding. Such instances remind us how numerous are the 

 bonds by which the lives of organisms are tied together. 



314/. I have reserved to the last the clearest and most 

 striking illustration of this progressing integration through- 

 out the organic world. I refer to the mutually-beneficial 

 relations established between plants and animals through the 

 agency of flowers and insects. 



Everyone nowadays has been made familiar with the pro- 

 cess of plant-fertilization, and knows that (leaving out of 

 consideration plants fertilized by wind-borne pollen) the 

 ability to bear seed depends largely on the aid given by bees, 

 butterflies, and moths. The exchange of services has been 

 growing ever more various and complicated during long past 

 periods. We have the acquirement by flowers of bright 

 colours serving to guide these insects to places where honey 

 is to be found; and we have their perfumes, also serving for 

 guidance. Then we have the many different arrangements, 

 often complicated, by which the visiting insects are obliged 

 to carry away pollen and dust with it the stigmas of flowers 

 on which they subsequently settle: thus effecting cross- 



