THE GENERAL SIGXIFICANCE OF AMPHI3IIXIS 199 



already attained? According' to the couception of the processes 

 within the germ-plasm which we have ah^ead}' stated, it is impossible 

 that this should be the case, for continual slight fluctuations are 

 occurring in the determinants in consequence of the fluctuations of 

 the nutritive stream, and these slight variations, plus or minus, do 

 not in inanj^ cases equalize one another or counteract one another 

 bv turning again in a contrary direction ; they go on increasing in 

 the direction in which they have begun. It is only when personal 

 selection opposes them that thej^ come to a standstill, and this can 

 only happen when they attain to selection-value, that is to say, when 

 the}^ reach a level at which they become disadvantageous in the 

 struggle of persons. But as germinal variations of this kind are 

 continually occurring, personal selection must keep continual watch 

 over them, and eradicate them as soon as the}^ have attained selection- 

 value. 



Therefore, when a species is most perfect!}' adapted to its 

 conditions, it would of necessity begin to degenerate if personal ^ 

 selection were not continually guarding it, and setting aside every- 

 thing that is in excess or deficient as soon as it begins to be 

 prejudicial. But the adaptation of a species does not depend upon 

 one character persisting at its normal level, but on the persistence of 

 very many, and man}' of these vary simultaneously upwards or 

 downwards, and reach the limit of selection-value at one time or 

 another. If there were no amphimixis, then either all individuals 

 with any excessive variant would be at once eliminated, or the species 

 would go on deteriorating until this excessive variant was so 

 numerously and strongly represented in all its individuals that it 

 would perish through degeneration. But even in the first of these 

 cases the species would drift towards the fate of extinction, because 

 excessive variations do occur even in every asexual generation, and 

 would appear in an increasingl}' large number of determinants if 

 there were no possibilit}' of rejecting them and eliminating them 

 from the lineage of the species. 



This is made j)ossil)le through the periodic intervention of 

 amphimixis ; it is actually effected thereby : and in this way alone 

 the species is kept at its high- water mark of adaptation. It is not 

 necessary to assume tliat every single determinant which is varying 

 in an unfavourable direction is at once eliminated as soon as it 

 Vtecomes prejudicial, that is, reaches negative selection-value, or — to 

 make use of an expression introduced by Amnion — as soon as it ^ 

 oversteps the boundaries of the ' playgTOund of variations,' the limits 

 within which variations are neither favourable nor unfavourable. But 



