REPRODUCTION 343 



of the epidermis which are constantly being formed to 

 make up for those shed, or it may be a fmiction called 

 into play only for the purpose of filHng up the ranks in 

 tissues which have been destroyed by disease. Such a 

 process happens in the lung epithehum after pneumonia. 



Death of the individual in the higher animals corresponds 

 to the cessation of the capacity to divide in protozoa ; 

 and, as in the latter, the continuation of the race depends 

 upon a periodic fusion of cells. But fusion as it occurs 

 in the protozoa differs from :^usion in higher forms in two 

 important respects. First, in protozoa all the cells pro-, 

 duced by division seem to be capable of pairing and fusing, 

 while in the higher animals this capacity becomes the 

 special property of a small group of cells which exist for 

 no other purpose. As we ascend in the animal scale 

 these cells become fewer relatively to the whole body. 

 Secondly, while in the protozoa the two cells which fuse to- 

 gether appear to have identical structure, in higher forms 

 a difference arises between them. This is associated mth 

 anatomical differences in the two individuals in which 

 they are formed. Of these individuals one plays but a 

 momentary part in the process of reproduction, while the 

 other protects and nourishes the offspring until the latter 

 is capable of an independent existence. The higher the 

 animal the more prolonged is the period of its helpless- 

 ness. The changes which take place in the reproductive 

 process during evolution may therefore be summarised 

 as a speciahsation in certain cells of the capacity for fusion, 

 differentiation of sex and increasing dependence of the 

 young upon the mother. 



Yet though in higher animals the sexual organs have 

 sunk to form but a small part of the body, they exert a 

 profound influence upon the growth and metabohsm of the 

 whole organism. We shall see how the ovary and testis 

 pour into the blood substances the presence of which is 

 necessary for the orderly succession of events which make 

 up the reproductive process, beginning in the desire for 



