THE EVOLUTION OF SEX 127 



all the elaborate devices for securing pollination, and by far the 

 most active part in the process is played by external agencies, 

 especially by those insects which have become the vicarious 

 fertilizers of the flowers. 



In the higher animals, on the other hand, the necessity for 

 bringing the~ganiete's into close proximity with one another has 

 led to the development of all those secondary sexual characters, both 

 bodily and mental, which play so conspicuous a part in the drama 

 of life. Throughout the whole course of this remarkable process 

 of evolution, except in the case of certain obviously degenerate 

 forms, we observe that same fundamental distinction between the 

 sexes which we first noticed in the gametes of unicellular 

 organisms, and which in the higher animals is extended with 

 the sexual differentiation itself from the gametes to the complex 

 multicellular body which bears them. The female is the more 

 passive partner and is especially concerned with the nutrition 

 and rearing of the offspring, and her bodily organization is 

 especially adapted to her maternal functions. These functions 

 constitute an inevitable handicap in the struggle for existence, 

 and the females and young of the higher animals are in most 

 cases largely dependent upon the less burdened and consequently 

 more active and vigorous males for their protection. 



The explanation of this progressive sexual differentiation is 

 undoubtedly to be found in the advantages to be derived from 

 division of labour and the accompanying possibilities of special- 

 ization. The origin of conjugation itself, upon which all sexual 

 phenomena are based, is another, and more fundamental, questioriSw 

 At first, as we have seen, the conjugating gametes were apparently ^ 

 exactly alike one another and exhibited no visible sexual differen- 

 tiation at all. The habit of conjugation probably arose from 

 the necessity of making good some disturbance of equilibrium in 

 the protoplasm of the cell. It has been supposed that, as the result 

 of repeated fission, some condition of inequality was gradually 

 set up amongst the daughter cells, whereby some of them came 

 to have too much of one constituent and too little of another, 

 while others were in the opposite condition. In this way the 

 successive unicellular generations gradually became more and 

 more enfeebled as we saw in the case of Paramoecium and, owing 

 perhaps to some sort of polarization, those which had become 

 modified in opposite directions came to exercise an attraction 

 upon one another which resulted in conjugation and restoration 



