156 LECTUEES ON BIOLOGY 



abundance of food, a more rapid increase in the number of 

 their natural enemies. Then the reverse process begins. The 

 caterpillars and butterflies, threatened by numerous enemies, are 

 rapidly reduced in number ; their enemies, consequently, suffer 

 want of food, and their increase becomes in turn restricted ; 

 thus the pendulum swings to and fro. 



But it is not always easy to determine the factors that main- 

 tain the balance, as is proved by Darwin's classic instance, 

 which showed that the fertility of the daisy and red clover in 

 any district is determined by the number of cats in that district. 

 It is well known that insects play an important part in the 

 fertilization of many flowers as carriers of pollen. Indeed, many 

 flowers have so much adapted themselves to certain species of 

 insects and their visits that only these can effect fertilization. 

 Without fertilization there can be no seed. According to 

 Darwin the pollination of red clover and daisies is exclusively 

 effected by the humble-bee, because only these insects are able, 

 thanks to their long proboscis, to reach the object of their 

 desire, the nectar. Thus a hundred clover plants yielded under 

 normal conditions two thousand seven hundred seeds, whilst a 

 like number, which had been protected against the visits of the 

 humble-bee, did not yield a single seed. The number of the 

 humble-bees of a district depends chiefly upon the number of 

 field-mice which on their subterranean wanderings rob and 

 destroy many nests. Finally, cats are probably the most zealous 

 persecutors of field-mice, and their number is, therefore, the 

 determining factor in the number of field-mice. There are, of 

 course, other enemies, the mouse-buzzards, owls, storks, &c., 

 who play a not unimportant part, but in this way the fertility 

 of clover can, nevertheless, be traced directly to the number 

 of cats. 



Though these instances show that the various species of 

 animals exercise a controlling effect upon each other, this form 

 of the struggle for existence is not that to which Darwin ascribes 

 the rdle of the natural breeder. Natural selection can only take 

 place if among the numerous descendants of one species 

 generally those individuals survive, reach maturity, and transmit 

 their individual characteristics to their offspring, which in the 

 structure of their body are best fitted to the conditions of life, 



