PERSISTENCE OP AMPHIMIXIS 97 



plant are so stable that they can thus remain unafiected by com- 

 plete changes of environment. 



This remark is intended as an introduction to a consideration 

 of the biological importance of amphimixis. Amphimixis con- 

 sists in the meeting and mingling of two nuclei. We cannot say 

 of amphimixis that it is necessarily bound up with multipUcation ; 

 for in the conjugation of two unicellulars, which is the lowest 

 form of amphimixis, the immediate result is that two individuals 

 become one. This amalgamation is, indeed, followed by fission, 

 but cannot be said to be the cause of it, as fission is of frequent 

 occurrence without any preceding conjugation. Indeed, con- 

 jugation often occurs at long intervals in the series of successive 

 fissions. 



Nor can we say of amphimixis that it has, not merely a 

 fertihsrng, but also a " rejuvenating " effect ; and that without 

 it a species must, in the long run, die out in a sort of sexual 

 decrepitude. The experiments of Weismann appear to contradict 

 the theory of Maupas on this point. But that amphimixis is 

 in some way of the highest importance is shown by the fact that 

 it is associated with reproduction in the great majority of cases 

 throughout the entire organic world. 



The fact which we have mentioned above, as to the relation 

 between the age of a character and its power of persistence, gives 

 us the clue to the extraordinary persistence of amphimixis 

 throughout the animal world. Those determinants of the germ- 

 plasm which determine this particular character of the sex-cells 

 must be the most obstinately persistent determinants in the 

 germ-plasm of the species — ^more persistent even than the deter- 

 minants of specific characters themselves ; for amphimixis is 

 more ancient than the species of to-day, and was probably char- 

 acteristic of their earliest ancestors. So that, even if natural 

 selection did not continue operative in securing the persistence 

 of this phenomenon, we can still understand how it is possible 

 for amphimixis to have persisted through hundreds of thousands 



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