IN THE THEORY OF NATURAL SELECTION. 293 



and has been itself largely removed by denudation, since that 

 time. 



Now if this theory as to the, causes of deterioration in disused 

 organs be correct, it follows that rudimentary organs can only 

 occur in species with sexual reproduction, and that they cannot be 

 formed in species which are exclusively reproduced by the partheno- 

 genetic method: for, according to my theory, variability depends 

 upon sexual reproduction, while the deterioration of an organ when 

 disused, no less than its improvement when in use, depends upon 

 variability. There are therefore two reasons which lead us to 

 expect that organs which are no longer used will remain un- 

 reduced in species with asexual reproduction : first, because only 

 a very slight degree of hereditary variability can be present, viz. 

 such a degree as was transmitted from the time when sexual 

 reproduction was first abandoned by the ancestors ; and, secondly, 

 because even these slight degrees of variability are not combined, 

 or, in other words, because panmixia cannot occur. 



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 uni- 

 sexuality of the majority cannot have been of very long diiration. 

 The process of transformation of the species from a bisexual into 

 a unisexual form, only composed of females, is obviously in- 

 complete, 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 proportion of males is the general rule, and is not confined to 

 single colonies. Thus Adler 1 counted 7 males and 664 females in 

 the common Cynips of the rose. 



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



