THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 195 



SO endlessly diverse in form and so inconceivably complex, could 

 not have taken place. 



But I do not regard this amphimixis as the real root of variation 

 itself, for that must depend not on a mere exchange of ids, but 

 rather upon a variation of the ids. The ids of a worm of the primitive 

 world could not without variation now make up the germ-plasm 

 of an elephant, even if it be true that mammals are descended from 

 worms. The ids must have been meanwhile transformed times without 

 number by the modification, degeneration, and new formation of deter- 

 minants. Amphimixis, that is, the union of two germ-plasms_, does not 

 •of itself cause variation of the determinants, it only arranges the 

 ids (the ancestral plasms) in ever-new combinations. If the origin 

 of variation were limited to that alone, a transmutation of species 

 and genera would only be possible on a very limited scale; there could 

 at most be a narrow circle of variations, just as in the example 

 already given of the packs of cards ; even if the taking away and 

 mixing up of the halves were repeated a thousand times, a definite 

 though undoubtedly large number of card-combinations would in the 

 long run recur again and again. But the case is difierent with the germ- 

 plasm and amphimixis, where there is an infinitely more varied series 

 of results, because the individual cards — the ids — are variable, even 

 between one time of sifting and shufiling and another, and therefore 

 infinitely productive of vai'iety in the course of numerous repetitions 

 ■of the shufliing. 



I have been frequently and persistently credited with maintain- 

 ing that the germ-plasm is invariable — a misunderstanding of my 

 position, due perhaps to a somewhat too brief and terse statement 

 which I made at an earlier period (1886). I had described the germ- 

 plasm as ' a substance of great power of persistence,' and as varying 

 with difiiculty and slowly, basing this statement upon the age-long 

 persistence of many species in which the specific constitution 

 of the germ-plasm must have remained unchanged. The idea of 

 'germinal selection,' of a ceaseless struggle between the 'primary 

 constituents' of the germ, and of the resulting continual slight and 

 invisible rising and falling of individual characters, had not yet 

 ■dawned upon me, nor had I at that time formulated the conception 

 ■of ' determinants.' I was even doubtful at that time whether develop- 

 ment, heredity, and variation were not interpretable on the assumption 

 of an undifferentiated substance without primary constituents. But 

 at no time was I unaware that the whole phyletic evolution of the 

 organic world is only conceivable on the assumption of continual 

 variation of the germ-plasm, that it actually depends upon this, even 



a 



Digitized by Microsoft® 



