44 *THE CAUSATION OF DISEASE. 



question, " Why does gamogenesis occur?" and to this he 

 replies, that "The approach towards general equilibrium in 

 organisms is accompanied by an approach towards molecular 

 equilibrium in them ; and that the need for this union of 

 sperm-cell and germ-cell is the need for overthrowing this 

 equilibrium, and re-establishing active molecular change in the 

 detached germ — a result which is probably effected by mixing 

 the slightly different ' physiological units ' of slightly different 

 individuals."* It would be quite beside our purpose to discuss 

 the question here ; but I imagine that both these great philoso- 

 phers are hovering near the mark, and that, although the 

 phraseology may eventually be altered, the whole question will 

 eventually be found to turn on ' ; equilibrium." 



We have already seen that there is no sharp dividing line 

 between asexual and sexual reproduction. Each method is in 

 the last resort very similar. The question now occurs, Can we 

 make any generalization embracing the several methods of re- 

 production ? All the facts seem to point to the conclusion that 

 each part of an orgauism possesses a tendency to reproduce the 

 whole. The twig of a plant placed in the ground will grow into 

 the likeness of the mature organism. A siugle cell taken from 

 the leaf of the Begonian plant will reproduce the plant ; where- 

 fore in this plant the power of reproduction belongs impartially 

 to every part of the organism, and the same is true of very many 

 of the lower forms of plant life. The like tendency is observed 

 in the animal world, for if the hydra be divided into several 

 pieces, each will grow up into a separate and entire organism. 

 As the scale of structural complexity in the animal world is 

 ascended, however — that is to say, as the process of division 

 of labour proceeds, and groups of cells become " specialized " 

 for some one particular function, this power wanes — individual 

 •cells gradually losing the power of producing the entire organ- 

 ism, and the power of reproduction is confined to cells which 

 have not undergone this specialization — viz., to those formed 

 by the ovary and testicle. This is the view taken by Herbert 

 Spencer. Here are his exact words : — 



" The marvellous phenomena initiated by the meeting of sperm- 

 oall and germ-cell, naturally suggest the conception of some quite 



§ 92, vol. i. • 



