VARIATION 433 



absolute equality of nutrition in homologous determinants, either 

 in different individuals or in the different ids of the same iierm- 

 plasm, is inconceivable, and as every minute variation of a 

 determinant does not disappear of itself witli the individual in 

 which it is present, but is transmitted directly to the germ-plasm 

 of the next generation, there can be no scarcity of variations of 

 every determinant, and the presence of the material required 

 for all the possible variations of all parts seems to be supported 

 theoretically. 



Before considering the modifications which the germ-plasm 

 must undergo as a zuJiole during the transformation of species, 

 I should like to meet an objection which might be raised. If 

 all determinants are incessantly exposed to slight inequalities of 

 nutrition and consequently suffer slight variations, what is the 

 cause of the extraordinary pertinacity by means of which the 

 species is preserved without the type undergoing variation, — 

 luhat is the cause of the constajicy of species ? We might expect 

 that all organic forms must be in a constant state of transition, 

 and that no form, and no organ, could be retained for any length 

 of time. 



Several points are disregarded in such a question. In the 

 first place, every species is under the uninterrupted control of 

 natural selection, as is clearly shown by the degeneration of 

 parts which have become useless. And since the old hypothesis 

 of the transmission of somatic variations must, it appears to me, 

 be definitely rejected, this process of degeneration can only be 

 explained as the result of panmixia, i.e.^ the cessation of the 

 control of natural selection over that part which is no longer 

 of use. We may, however, conclude from the fact that such 

 degeneration is universal, that the determinants are in a con- 

 stant state of fluctuation ; and as degeneration takes place very 

 slowly in all cases, I further infer tliat, in spite of the frequency 

 with which these fluctuations occur, tJiey only increase very 

 gradually so as to give rise to perceptible variations. 



As at first stated, the individual fluctuations of the deter- 

 minants must be regarded as excessively small. Natural 

 selection could produce no direct result from an individual 

 variation, for it could not produce a cumulative effect ; an 

 accumulation can only be produced by amj^himixis, and I am 

 inclined to assume that much of the importance of the latter 

 process is due to this fact. It can cause minorities of modified 



