42 THE CAUSATION OF DISEASE. 



closely related, we may cite the fact that many alga? are repro- 

 duced by a process termed conjugation, in which a cell of one 

 alga unites with that of another, the united mass growing into 

 a separate organism. 



All the highest forms of life are reproduced by the sexuaL 

 method. Every naturalist must have asked himself why such 

 union of two distinct elements is necessary for the reproduction 

 of the higher forms of life. Now, respecting sexual reproduc- 

 tion, there are two conclusions to which we are easily led. First. 

 that the whole object of sexual reproduction is to secure the 

 union of two pieces of protoplasm derived from separate organ- 

 isms ; for it has been beautifully shown by Darwin, especially 

 in the case of plants, that organisms containing in themselves 

 both sexes are most cunningly supplied with appliances whereby 

 an occasional fertilization by another organism is ensured. 



Sexual reproduction, then, having for its object the union 

 of protoplasmic masses belonging to distinct individuals, 

 the question arises, why is this ? Can we make any further 

 generalization? Certainly we can. And this leads to the 

 second conclusion to which I referred — namely, that the 

 object of sexual reproduction is to secure the union of masses 

 of protoplasm belonging to separate unlike organisms of 

 the same species : if, that is to say, the separate organisms 

 were exactly alike, reproduction would not occur. This im- 

 portant generalization rests upon observation. In proportion 

 as the organisms coming into sexual union are like ono 

 another, in that same proportion is the propagation of the 

 species rendered difficult. Perpetual breeding in and in 

 leads to deterioration and eventual extinction of the species. 

 But in proportion as judicious crossing between different 

 varieties takes place, in that proportion is the stock im- 

 proved. " It is," says Darwin, " a great law of Nature that 

 all organic beings profit by an occasional cross with indi- 

 viduals not close!)/ related to them in blood, and that, on the 

 other hand, long-continued close inter-breeding is injurious.'"* 

 Darwin adduces irrefutable evidence in proof of this. He 

 reminds us that in this controversy the good resulting from 

 inter-crossing has not been so frequently discussed as the 

 * "Variation under Domestication," vol. ii. p. 94 (2nd edit, revised). 



