212 THE EVOLUTION THEORY 



for the persistence of any essentially different mode of reproduction 

 with quite different adaptations. Or, to speak metaphorically, the 

 power of adaptation which is innate in the organism so exhausted 

 itself in the establishment of this marvellous amphimixis adjustment 

 that the possibility of any other was totally excluded. 



It is true that it is only among the Vertebrates that we find 

 ' the reproductive apparatus ' so highly developed, but even among 

 Molluscs and Arthropods ' sexual ' reproduction, that is, reproduction 

 associated with amphimixis, is the prevailing mode. In these, indeed, 

 parthenogenesis does occasionally occur, that is to say, sexually 

 differentiated female germ-cells are, by means of some slight varia- 

 tions in the maturation of the agg, rendered capable of developing 

 without previous amphimixis, but this happens only in quite special 

 cases as an adaptation to quite special circumstances, and can only be 

 regarded as a temporary cessation of the association between repro- 

 duction and amphimixis. In some cases it is a moiety of the ova 

 adapted for amphimixis which develop parthenogenetically, as it 

 is the same sexually differentiated animals, true females, which 

 produce both sorts, and this is often true to some extent when the 

 differentiation in the direction of parthenogenesis has advanced 

 further, and the ova have been separated into those requiring ferti- 

 lization and those which are parthenogenetic (e.g. the winter and 

 the summer eggs of the Daphnida;). Parthenogenesis is not asexual 

 but unisexual reproduction, a mode of multiplication which shows 

 us that even in highly differentiated animals the apparently indis- 

 soluble association between reproduction and amphimixis can be 

 dissolved if circumstances require it. 



But if amphimixis had to be retained in the higher animal forms 

 — and we have seen reasons why this must be — it could only l)e 

 effected by means of unicellular germs, for amphimixis is in essence 

 a fusion of nuclei, and this is the reason why ' vegetative ' repro- 

 duction, so-called, becomes less and less prominent in animals at least, 

 and above the level of the Arthropods disappears almost entirely. 



Let us now return to the question we asked at the beginning — 

 When and in what form was amphimixis first introduced into the 

 world of organisms ? The best way to answer this is by observa- 

 tion. We must turn to the lowest forms which now exhibit it, and 

 see whether it occurs in them in a simpler form, so that we may draw 

 conclusions as to its origin and its primitive significance, for it would 

 be possible, a 2^'i^iori, that this was something different from what 

 it is now in the relatively higher organisms, and that a change 

 of function has gradually come about. 



