I02 HEREDITY AND VARIATION 



It means, in the case of unicellular organisms, that the sepa- 

 rated parts are identical in substance and carry on the complete 

 organisation of the parent cell in absolute integrity. In the case 

 of multicellular organisms it depends on the same thing. The 

 cell which in the embryo begins the germ-cell lineage may be 

 identical with the fertilised ovum, and the complete heritage may 

 be continued intact through successive cell-divisions until the 

 next generation is started, and the process begins anew. The 

 completeness of hereditary resemblances depends, in Bateson's 

 phrase, on " that qualitative symmetry characteristic of all non- 

 differentiating ceU-divisions." 



It seems, therefore, useful to say that variation is " the ex- 

 pression of a qualitative as5mimetry beginning in gametogenesis. 

 Variation is a novel cell-division." But to tell what specific 

 cause induces this novelty is still beyond our power. Yet we 

 can point to certain conditions which may induce novelty or 

 qualitative asymmetry in gametogenesis. Thus, there is the 

 complex changeful environment of the developing germ-cells, 

 there is the possible struggle of analogous hereditary units or 

 determinants for sustenance, there is the complex process of 

 reduction which occurs during the maturation of the germ-cells, 

 and there are the chances of new combinations and permutations 

 in fertilisation. 



Results of Amphimixis. — That amphimixis is one of the provocatives 

 of variations is strongly suggested by what results when two breeds 

 are interbred. As Prof. Cossar Ewart says * : " Domestic animals 

 reproduce themselves with great uniformity if kept apart ; but 

 the moment one mixed up two different races, strains, or breeds, 

 one did something that was difficult to put in words, but the 

 result was what has been best described as an ' epidemic ' of 

 variations." 



* Discussion on Heredity in Disease, Scpttish Med. and Surg. Journal, vi. 

 1900, p. 308. 



