v.] IN THE THEORY OF NATURAL SELECTION. 30T 



And the facts seem to point in the direction required by the 

 theory, for superfluous organs do not become rudimentary in 

 parthenogenetic species. For example, as far as my experience 

 goes, the receptaculum seminis does not deteriorate, although it 

 is, of course, altogether functionless when parthenogenesis has 

 become established. I do not attach much importance to the 

 fact that the Psychids and Solenobias — (genera of Lepidoptera 

 which Siebold and Leuckart have shown to include species 

 with parthenogenetic reproduction) — still retain the complete 

 female sexual apparatus, because colonies containing males 

 still occasionally occur in these species. Although the majority 

 of colonies are now purely female, the occasional appearance of 

 males points to the fact that the unisexuality of the majority 

 cannot have been of very long duration. The process of trans- 

 formation of the species from a bisexual into a unisexual form, 

 only composed of females, is obviously incomplete, and is still 

 in process of development. The case is similar with several 

 species of Cynipidae, which reproduce by the parthenogenetic 

 method. In these cases the occurrence of a very small pro- 

 portion of males is the general rule, and is not confined to 

 single colonies. Thus Adler^ counted 7 males and 664 females 

 in the common Cynips of the rose. 



In some Ostracodes, on the other hand, the males appear to 

 be entirely wanting : at least, I have tried in vain for years to 

 discover them in any locality or at any time of the year-. 



Cypris vidua and Cypris reptans are such species. Now, 

 although the transformation of these formerly bisexual species 

 into purely unisexual female species appears to be complete •\ 

 yet the females still possess a large, pear-shaped receptaculum 



1 Adler, ' Zeitschrift f. wiss. Zool.,' Bd. XXXV, 1881. 



^ Compare my paper, ' Parthenogenese bei den Ostracoden,' in 'Zool. 

 Anzeiger,' 1880, p. 82. Purely negative evidence, unless on an immense 

 scale, is quite rightly considered to be of no great value in most cases. 

 But the condition of these animals renders the accumulation of such 

 evidence unusual!}^ easy, because the presence of males in a colony of 

 Ostracodes can be proved by a very simple indirect test. Thus if a 

 colony contains any males, the recepfaaila seminis of all mature females 

 are filled with spermatozoa, and on the other hand we may be quite sure 

 that males are absent, if after the examination of many mature females, 

 no spermatozoa can be found in any of their reccptacnla. 



3 We cannot, however, be absolutely certain of this, for it is con- 

 ceivable that males may still occur in colonies other than those 

 examined. 



