THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 223 



might have thought that the vital procesnes would he thus more 

 readily recuperated than hy the co-operative coml dilation of two 

 already 'exhausted ' cells. Two exliausted horses may perliaps be able 

 to 2)ull the load that one of them was no longer equal to, but in the 

 case we are considering it is the combined burdens of two units that 

 have to be borne, although each was no longer equal to its own share I 

 That is more than we can understand. 



Zehnder has recently defined the effect of amphimixis as 

 a ' strengthening of the power of adaptation,' and he infers tliat tlie 

 'digestive fistella3' (Biophors) of two individuals, Avhich have some- 

 what different powers of digestion, are, when they combine, able 

 to assimilate more kinds of food than either was able to assimilate 

 by itself. But I confess that I do not see how an advantage for the 

 Avhole would be gained through this alone, since half of the digestive 

 biophors would have to work for the nutrition of the mass of the 

 individual A, the other half of the differently constituted biophors 

 for that of the individual B, and the nutritive capacity would thus 

 remain exactly what it was before conjugation. Nevertlieless I belie^'e 

 that Zehnder was right in his supposition that conjugation is con- 

 cerned with strengthening the power of adaptation, and I have long 

 maintained and defended this interpretation with regard to true 

 amphimixis in nucleated organisms. In these cases it is quite obvious 

 that the communication of fresh ids to the germ-plasm implies an 

 augmentation of the variational tendencies, and thus an increase 

 of the power of adaptation. Under certain circumstances this may 

 be of direct advantage to the individual which results from the 

 amphimixis, but in most cases the advantage will be only an indirect 

 one, which may not necessarily be apparent in the lifetime of tliis 

 one individual, but may become so only in the course of generations 

 and with the aid of selection. For amphimixis must bring together 

 fa^'Ourable as well as unfavourable variations, and the advantage 

 it has for the species lies simply in the fact that the latter are weeded 

 out in the struggle for existence, and that by repetition of the process 

 the unfavourable variational tendencies are gradually eliminated more 

 and more completely from the germ-plasm of the species. 



But this cannot have been the efficient cause in the introduction 

 of amphimixis into the series of vital phenomena ; the reason for this 

 must be found in some direct advantage, such as that it improved and 

 increased the assimilating power, the growth, and the multiplication 

 of the particular individual, so that it gained an advantage over 

 individuals which had not entered into conjugation. This advantage 

 mttst exist, at least in the lower forms of conjugation, in pure plasto- 



