THE CONSERVATION OF LIFE 229 



That this is, in fact, the meaning of amphimixis is shown by the 

 observation that only those animals proceed to conjugation which 

 are most distantly related. Further, a large number of accurate 

 investigations made with different species of infusorians have shown 

 that individuals descended from the same mother-animalcule did 

 not conjugate even when all other conditions of union have been 

 fulfilled. In fact, conjugation between closely related individuals 

 greatly resembling each other would lose its greatest object ; in 

 many cases it would even do greater harm than good by 

 accumulating like characters. It is the same cause which forbids 

 inbreeding of higher animals and matrimony between blood- 

 relatives. Just as Nature generally prevents self-fertilization 

 and inbreeding, so the human race, in true perception of the 

 dangers, created in early times laws and manners which limit or 

 entirely forbid marriage between parents and children, between 

 brothers and sisters, and even between cousins. 



The great resemblance of the conjugation-process of the 

 unicellular organisms to the fertilizing act of the higher animals 

 is remarkable. Fundamentally they are processes of the same 

 kind, and we are probably entitled to assume that conjugation 

 is the early phylogenetic stage from which the process of 

 fecundation was, in the course of phylogenetic development, 

 gradually evolved. We shall see later on that there are in the 

 organic world of to-day almost imperceptible transitions from the 

 ' union ' of Paramcecium to the phenomena of sexual reproduction 

 of the multi-cellular animals. We shall also learn when first the 

 phenomenon of death became an inexorable law, and we shall 

 further see how with progressive differentiations its power became 

 more and more established and extended until it appears finally 

 as a necessary stage of organic development. 



Though the conjugation-process of the infusorians and the fer- 

 tilization of multi-cellular organisms are of equal importance 

 they differ greatly in their external appearance. While in Para- 

 macium and its class the entire animalcules fuse in the ' sexual 

 act,' to separate again after the exchange of nuclear substance, 

 in the higher and highest organisms it is only the germ-cells, 

 definite minute particles separated from the body, containing the 

 heritable characters, which unite during the act of fertilization 

 and remain in a permanently close union. From this insignificant 



