DEGENERATE SEXUAL REPRODUCTION. 



163 



once over five thousand in number, without finding a single male. At other 

 times he found one per cent., while in certain unknown conditions (probably 

 when food is scarce and life generally unfavorable) the males may be developed 

 in crowds. 



In the daphnids, which have been so successfully studied by Weismann, the 

 facts are more complex. There are two kinds of eggs — winter and summer 

 ova. The former are large, thick-shelled, capable of resisting drought and the 

 like, and of remaining leng latent. They only develop if fertilized, and always 

 produce females. In every way they are highly ana- 

 bolic ova. The summer eggs, on~the other hand, are 

 smaller, and thinner in the shell. They can develop 

 without fertilization, and that is indeed in some cases 

 physically impossible. Males are produced from sum 

 mer eggs alone. They usually appear in autumn, when 

 life is becoming harder, or the conditions more kata- 

 bolic. 



In the little cyprids the reproductive relations are 

 very varied. Thus in Cypris ovum and Notodromus 

 monachus the males are abundant all the year round, 

 and parthenogenesis is unknown. In other species — 

 for example, Candona Candida — the males are still 

 frequent, but parthenogenesis nevertheless occurs. 

 Lastly, parthenogenesis prevails in some cases, like 

 Cypris fusca and C. pubera, and the males are rare, 

 appearing usually in spring. 



(c) In insects, as we have seen, the degrees of 

 parthenogenesis are very varied ; so, too, is the 

 systematic position of the forms in which normal 

 parthenogenesis occurs. Two butterflies {Psyche helix 

 and Solcnobia, 2 sp.) and a beetle (Gaslrophysa); 

 some coccus-insects and aphides ; certain saw-flies 

 ( Tenthredinida) and gall-wasps ( Cynipidai), are nor- 

 mally parthenogenetic. In the butterflies just noticed, 

 the males seem to disappear for a stretch of years, 

 and the species gets on without them. The male of 

 Psyche helix is very rare, and was for long unknown. 

 When the males are developed in Solenobia trinque- 

 Irella, it is interesting to notice that they may predom- 

 inate in numbers over the females. A whole brood 

 may be male ; they are brought back with a rush. 

 About a score of moths, including the silkmoth (Bom- 

 byx ntori) and death's-head {Sphinx alropos) have 

 been known to exhibit casual parthenogenesis ; but 

 the beetle above noticed stands alone. Bassett, Adler, 

 and others, have demonstrated an interesting alternation of parthenogenesis 

 and ordinary sexual reproduction in numerous gall-wasps. Forms which had 

 been regarded as quite distinct, and had received different generic titles, have 

 been shown in about a score of cases to be merely the parthenogenetic and 

 normal forms of the same insects. From a winter-gall the parthenogenetic form 

 emerges which produces a summer-gall. In this a sexual form is produced, 

 which eventually gives rise to the winter-gall. . 



Fig. 49. — Owen's figure of the 

 Generations of Aphides. 

 At the base an individual 

 arises from a fertilized egg- 

 cell ; this gives origin par- 

 theno- genetically to a 

 brood, and so on through 

 a succession of genera- 

 tions. At the top the male 

 and female forms reappear, 

 and sexual reproduction 

 returns. At the side an 

 earlier appearance of sex- 

 ual forms is suggested. 



