BLASTOGENIC VARIATIONS. 103 



as, by a process of accumulation, they form the material 

 from which the visible individual variations are pro- 

 duced by means of " amphimixis." 



Amphimixis is that form of reproduction which is 

 found in all the higher organisms, and which consists in 

 the mingling of two individuals or their germs; i. e., 

 the so-called sexual reproduction. The term is also ap- 

 plied to a similar phenomenon occurring amongst uni- 

 cellular organisms, i. e., to conjugation. In this case 

 reproduction is not a necessary or even usual concomi- 

 tant, but takes place independently. To amphimixis 

 Weismann attributes the constant occurrence of indi- 

 vidual variability, although he recognises that it is not 

 the primary cause of this variability; but rather the 

 process furnishes an inexhaustible supply of fresh com- 

 binations of individual variations. Thus the germ- 

 plasm of a new individual produced by amphimixis 

 never receives more than half the ids of each parent, 

 and these are differently selected and arranged in each 

 case. By an id, it may be remarked, Weismann means 

 a group of determinants which contains all the deter- 

 mining elements of the species, though in a manner 

 peculiar to the individual. 



Blastogenic variations are thus, according to Weis- 

 mann, primarily dependent on two chief factors: (1) 

 Inequalities of nutrition acting on the individual con- 

 stituents of the germ-plasm; (2) Amphimixis. 



It behoves us to examine these two factors more 

 closely, and see how far they are supported by experi- 

 mental evidence. It is, on the face of it, impossible to 

 put Weismann's hypothesis of the reaction of determi- 

 nants to inequalities of nutrition to a practical test, but 



