34 PROBLEMS OF EVOLUTION 



exact counterparts one of the other, maintains the conservative 

 reproductive cells. But it can only very rarely happen that the 

 two halves are absolutely exact counterparts, and on this fact, 

 as a foundation, we can build important conclusions. Among 

 bacteria, the only organisms among which, as far as is known, 

 conjugation does not take place, we seem driven to consider 

 this inequality of fission as the sole cause of variation. 1 The 

 environment I cannot look upon as a cause, but merely as a 

 condition to which the organism reacts a subject for the next 

 chapter. There is amphimixis : if we grant that it may be 

 the primary cause of variation, yet, if the two beings that 

 fuse are identical in character, it must lose much of its efficacy. 

 If, however, owing to unequal fission a certain amount of 

 differentiation had once arisen, then unions between individuals 

 would lead to an enormous variety of combinations. Weismann 

 has himself suggested that the first division of the nucleus 

 of the egg of a Metazoon may have for its object a rearrange- 

 ment of inherited characters. But I do not know that he 

 has applied the same principle to the fission of one-celled 

 organisms. Amphimixis he has declared to be the primary 

 cause of variation. No doubt amphimixis involves a rearrange- 

 ment of the particles of germ-plasm and so the union of two 

 exactly similar individuals may start variations. But, as is 

 well known, it gains its main efficiency in this respect, from 

 the fact that the two uniting individuals are not exactly 

 similar : every union is more or less of the nature of a 

 cross between two breeds. There is a preliminary dissimi- 

 larity, and fission, there is reason to believe, is able to put 

 this at the service of amphimixis, the union of two slightly 

 different cells leading to new combinations and at the same 

 time preventing the complete loss of indispensable characters, 

 one cell supplying the deficiencies of the other. Many evolu- 

 tionists in discussing the origin of variations make no mention 



1 Mr Adam Sedgwick, F.R.S., in his address to the Zoological Section of the 

 British Association, 1899, said: "Among bacteria alone, so far as I know, has the 

 phenomenon of conjugation never been observed." Some authorities, however, deny 

 it in the case of amoebae. 



