THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 229 



strongly attract each other of themselves, but they were equipped 

 with a strong power of mutual attraction, because the union of very 

 difterent individualities was the more advantageous. 



This is an important distinction, for the adaptation to allogamy is 

 widely distributed, and its latest manifestations have frequently been 

 misunderstood in the same way as its beginnings. The widespread 

 occurrence of allogamy has been interpreted as evidence in favour of 

 the rejuvenation theory, and the endeavour on Nature's part to secure 

 the union of the unlike has been associated with the hypothetical 

 ' rejuvenating ' power of amphimixis, and regarded as a direct and 

 inevitable outcome of this. That this view is erroneous we shall see 

 even moi'e clearly from what follows. 



As among unicellular Algai it is frequently only gametes of 

 different lineage which conjugate, so among animals and plants there 

 are numerous cases in which the union of nearly related gametes is 

 more or less strictly excluded, both by the prevention of self-fertiliza- 

 tion (autogamy) in hermaphrodites, or by the prevention of inbreeding, 

 that is, the continued pairing of near relatives. Now all the 

 preventive measures which efiect this are of a secondary nature ; they 

 are adaptations which result from the advantage involved in the 

 mingling of unrelated germ-plasms, even though it sometimes seems 

 as if they were an outcome of the primary nature of the 

 germ-cells. 



The primary result of the mutual chemical influence of the two 

 germ-cells upon one another is — apart from the impulse to develop- 

 ment which the centrosphere of the sperm-cell supplies — as far as 

 I see, only the more favourable or the more unfavourable mingling of 

 the biophor- or determinant-variants, and the resulting increase or 

 decrease in adaptive capacity, which leads to the better thriving of 

 the offspring, or conversely to its degeneration. Everything else is 

 secondary and depends upon adaptation, effected in very diverse ways^ 

 to secure the most favourable mingling of the germ-plasms for the 

 particular species concerned. Undoubtedly the parental ids united 

 through amphimixis have an effect upon each other, since throughout 

 the building up of the organism of the child the homologous deter- 

 minants struggle with one another for food, but they do not affect 

 each other in the way that many prominent physiological and medical 

 writers suppose, namely, that the union of the parental germ-plasms 

 sets up a ' formative stimulus ' which ' advances ' or even ' greatly 

 advances ' the process of development in the egg. 



Parthenogenetic development goes on just as rapidly, sometimes 

 even more rapidly than that of the fertilized ova of the same species ! 



