60 EVOLUTION OF LIVING ORGANISMS. 



male. Further, Balzer has recently given evidence that 

 the larvae of the strange worm-like animal Bonellia become 

 either female or male according as they live an inde- 

 pendent life or become parasitically attached to their 

 mother. The difficult problem of the determination of 

 sex cannot yet be considered as solved. The evidence 

 seems to show that the sex of an individual is fixed in 

 different ways and at different times. Some animals lay 

 male-producing and female-producing eggs ; among the 

 Hymenopterous insects, such as the bee, the egg produces 

 a female or a male according as it is or is not fertilised ; 

 while in other cases, as mentioned above, sex is deter- 

 mined by the kind of gamete which fertilises the ovum. 

 Moreover, although in the vast majority of cases the sex 

 of an individual appears to be irrevocably fixed at or 

 before fertilisation, yet there is evidence that in certain 

 cases it is influenced by environmental conditions. 



CHAPTER V 



THE STRUGGLE FOR EXISTENCE AND NATURAL SELECTION 



OF all the primary factors of evolution the struggle for 

 existence is that which lends itself least to controversy. 

 When once stated and understood it must be recognised. 

 Unfortunately, it is not always understood. The phrase 

 is a metaphor to express the undeniable fact that more 

 organisms are born into the world than can survive in it. 

 As Darwin says, unless "the truth of the universal 

 struggle for life be constantly borne in mind the whole 

 economy of nature, with every fact on distribution, 

 rarity, abundance, extinction, and variation, will be dimly 

 seen or quite misunderstood." The enormous rate at 

 which even the most slowly reproducing creatures are 

 capable of increasing is not always fully realised. Organ- 



