200 THE EVOLUTION THEORY 



in the course of generations they are unfailingly eliminated, especially 

 when a large number of unfavourably varying determinants are 

 coincident in the germ-plasm. Then the individuals which arise 

 from a germ -plasm thus composed must pei'ish in the struggle for 

 existence, and thus the id-combinations with excessive determinants 

 are eliminated from the germinal constitution of the species. As 

 this is repeated as often as excesses of the ids occur, the species 

 is kept pure. 



It might be objected that, through such a continual weeding- 

 out of rebellious determinants, the germ-plasm would become so 

 constant in its constitution that it would ultimately be secure from 

 all such aberrations of it on the part of its determinants, and therefore 

 would in time become quite incapable of diverging from its proper 

 path at all, and would thus no longer require this continual correction 

 through amphimixis. 



I do not wish to contradict this conclusion ; indeed, I believe 

 that the constitution of the species becomes more and more constant 

 in the way I have indicated, and that an ever more perfect and stable 

 equilil)rium of the whole determinant system is thus brought about. 

 It follows that in the course of generations the diverse determinants 

 of the germ-plasm will vary within a progressively shortened radius, 

 and will thus more and more rarely overstep the limits of the 

 ' variation-playground ' — and yet I still believe that this justifiable 

 conclusion tells in favour of my interpretation of the utility of the 

 persistence of amphigony once introduced. 



Let it be remarked, in the first place, that it is by no means 

 essential to the preservation of a useful institution that it should 

 practically justify its utility in everij generation. Although, for 

 instance, the warm winter coat of a species of mammal may be 

 necessary to its survival, it does not disappear at once when a winter 

 happens to occur which is so warm that even individuals with poor 

 pellage can survive. Indeed, several such mild winters might occur 

 in succession, in which there was no weeding-out of the individuals 

 with poor fur, and yet the thickness of the winter fur of the species 

 would not become less fixed, just because this character no longer 

 varies perceptibly in an old-established species which has long been 

 perfectly adapted, and it could only be brought into a state of marked 

 fluctuation very slowly through direct influence on the germ-plasm, 

 or through panmixia. But exactly the same tiling is true in regard 

 to the determinants of the reproductive cells, in respect of their 

 adaptation to amphimixis, only very much more emphatically. 



Before going further, I sliould like to show that the conclusion 



