SIGNIFICANCE OF SEX 321 



however, does not assist us to understand why sexual repro- 

 duction, the maintenance of which throughout the vegetable 

 kingdom forces us to believe in its usefulness, should be 

 necessary also. We here enter one of the most difficult 

 regions of biology, for, as we have said, we do not really 

 know what is implied in the sexual process. 



{a) The ovum is a reproductive body which normally 

 can develop only after a suitable stimulus has been applied 

 to it. This stimulus is normally the entrance of the sperm. 

 If the essence of sexual reproduction lies merely in a stimu- 

 lation to development, it is not clear what advantage the 

 organism, or the race, gains in clinging to a practice as 

 precarious as it is expensive. Further, the stimulus is not 

 of so highly specialised a character as to make essential the 

 co-operation of the second gamete. Parthenogenesis — the 

 development of an unfertilised egg — is a widespread pheno- 

 menon in the animal kingdom, and also occurs among 

 plants. Here the normal development of the egg cell is 

 determined by some stimulus or condition which, whatever 

 its nature, is not that of fertilisation. Further, artificial 

 parthenogenesis has been brought about both in plants and 

 animals by subjecting the gamete to a variety of abnormal 

 external influences, such as wounding, the action of salt 

 solutions and of organic acids. The stimulation of fer- 

 tilisation is not therefore unique^ — it may be due to some 

 simple effect, such as the admission of oxygen — nor does 

 it in any case explain the biological significance of sexual 

 reproduction. 



(b) It has been suggested that after a certain span of 

 vegetative existence the organism becomes weakened or 

 worn out ; the span of life is, in fact, definitely limited in 

 some plants as in many animals. It is possible that the 

 process of sexual fusion is in some way the means of rejuvena- 

 tion, that it starts the new generation of individuals with an 

 equipment of revived protoplasm. How this should come 

 about by the union of two exhausted cells is not easy to see. 

 The theory is strengthened if it is made broader, and the 

 increased vitality is made dependent on the union of gametes 



y 



