THE ORIGIN OF VARIATIONS 57 



aliases — plays a leading part in all theories of 

 heredity and variation. This conception, combined 

 with certain of the abstract conclusions reached in 

 "First Principles," enables Spencer to account for 

 the "spontaneous variation" of new individuals 

 simultaneously produced by the same parent — ani- 

 mals of the same litter. Plainly this is the case 

 which most severely tests any proti'ered explanation 

 of the causes of variation. 



Now Spencer had already enunciated a law termed 

 by him the law of the instability of the homogeneous. 

 In ordinary language this law may be interpreted 

 as stating that any homogeneous aggregate neces- 

 sarily tends to become heterogeneous, since its 

 several (similar) parts are necessarily exposed to 

 different forces, and are therefore of necessity differ- 

 ently modified. Spencer illustrates this law from 

 astronomy, geology, sociology, biology, and psycho- 

 logy. Now, if we apply the law of the instability 

 of the homogeneous to the mother-germ-cell, which, 

 by the process called gametogenesis — the genesis of 

 gametes — divides and gives rise to the actual gametes, 

 or sex-cells, we see that, since no two parts of any 

 aggregate are subject to precisely the same forces, 

 and must therefore become dissimilar, no two ova, 

 spermatozoa, or pollen-cells, formed as they are by 

 a process of evolution from the mother-germ-cell, 

 can he identical. Between them there must be 

 "small initial differences in the proportions and 

 condition of the slightly unlike physiological units." 

 At once we see that the view of Weismann had been 

 anticipated. The "small initial differences" in the 

 "physiological units" — or "determinants" — arise in 



