REPRODUCTION 57 



vent self-fertilisation and help cross-fertilisation are so 

 numerous that there can be no reasonable doubt that 

 nature abhors perpetual self -fertilisation. 



In the animal kingdom the occurrence of hermaphrodite 

 forms, forms in which both male and female reproductive 

 organs are found, is much more restricted than among 

 plants. Further, such hermaphroditic forms are seldom 

 self -fertilising, except in the case of parasitic forms where 

 cross-fertilisation is next to impossible. Thus we see 

 that the demand for cross-fertilisation becomes more 

 emphatic the higher we ascend in the animal kingdom. 

 Low down in the animal scale sexual differentiation 

 arises, and parthenogenesis tends to disappear, and in 

 like manner gemmation or budding dies out. Repro- 

 duction, then, among animals soon narrows down to the 

 sexual method ; this is to the liberation of special cells 

 from different individuals, the fusion of these cells, and 

 the development from the product of the fusion of a 

 larva or embryo, which gradually develops into a sexual 

 individual generally resembling one or other of the 

 parents. 



Among animals there must be therefore an early 

 differentiation of body or somatic cells and reproductive 

 or germinal cells. The latter are contained in special 

 organs until such time as sexual maturity awakens them 

 to activity, when they undergo certain changes which 

 are an essential preliminary to fertilisation. 



In plants the germinal cells do not differ much from 

 the somatic, and they appear only when reproduction 

 is to take place, and though passing through a prepara- 

 tion for fertilisation practically identical with that of 



