SEXUAL SELECTION 337 



has captured hundreds of the female Apatania muUebris, 

 but has never seen the male; and of Boreus hyemalis only- 

 four or five males have been seen here.*" With most of 

 these species (excepting the Tenthredinse) there is at present 

 no evidence that the females are subject to parthenogenesis; 

 and thus we see how ignorant we are of the causes of the 

 apparent discrepancy in the proportion of the two sexes. 



In the other classes of the Articulata I have been able 

 to collect still less information. With Spiders, Mr. Black- 

 wall, who has carefully attended to this class during many 

 years, writes to me that the males, from their more erratic 

 habits, are more commonly seen, and therefore appear more 

 numerous. This is actually the case with a few species; 

 but he mentions several species in sis genera, in which 

 the females appear to be much more numerous than the 

 males." The small size of the males in comparison with 

 the females (a peculiarity which is sometimes carried to an 

 extreme degree), and their widely different appearance, may 

 account in some instances for their rarity in collections.'" 



Some of the lower Crustaceans are able to propagate their 

 kind asexually, and this will account for the extreme rarity 

 of the males ; thus Von Siebold" carefully examined no less 

 than 18,000 specimens of Apus from twenty-one localities, 

 and among these he found only 319 males. With some 

 other forms (as Tanais and Cypris), as Fritz Miiller informs 

 me, there is reason to believe that the males are much 

 shorter lived than the females; and this would explain 

 their scarcity, supposing the two sexes to be at first equal 

 in number. On the other hand, Miiller has invariably 

 taken far more males than females of the Diastylidse and 

 of Cypridina on the shores of Brazil; thus with a species 



•e "Proo. Bnt. 8oe. London," Feb. 17, 1868. 



" Another great anthority with respect to this class. Prof. Thorell of Upsala 

 ("On European Spiders," 1869-70, part i. p. 206) speaks as if female spiders 

 were generally commoner than the males. 



"* See, on this subject, Mr. 0. P. Cambridge, as q'uoted in "Quarterly Jour- 

 nal of Science," 1868, p. 429. 



»" "Beitrage zur Parthenogenesia," p. 174. 

 Descent — ^Vol. I. — 15 



