462 GENERAL RESULTS. Chap. XIL 



chlorine, have a very feeble affinity for each other, 

 yet it cannot be said that the weakness of the affinity 

 depends in such cases on the extent to which the 

 substances differ. It is not known why a certain 

 amount of differentiation is necessary or favourable 

 for the chemical affinity or union of two substances, 

 any more than for the fertilisation or union of two 

 organisms. 



Mr. Herbert Spencer has discussed this whole subject 

 at great length, and after stating that all the forces 

 throughout nature tend towards an equilibrium, 

 remarks, " that the need of 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." * 

 But we must not allow this highly generalised view, 

 or the analogy of chemical affinity, to conceal from us 

 our ignorance. We do not know what is the nature or 

 degree of the differentiation in the sexual elements 

 which is favourable for union, and what is injurious for 

 union, as in the case of distinct species. We cannot 

 say why the individuals of certain species profit greatly, 

 and others very little by being crossed. There are some 

 few species which have been self-fertilised for a vast 

 number of generations, and yet are vigorous enough 

 to compete successfully with a host of surrounding 

 plants. Highly self-fertile varieties sometimes arise 



* 'Principles of Biology,' vol. i. changes in the conditions and 

 p. 274, 1864. In my 'Origin of from crossing widely distinct forms 

 Species,' published in 1859, I (i.e., species), as a series of facts 

 spoke of the good effects from " connected together by some- 

 slight changes in the conditions of common but unknown bond, 

 life and from cross-fertilisation, which is essentially related to 

 and of the evil effects from great the principle of life." 



