C «AP. VIII. 
SEXUAL SELECTION. 
315 
a nd therefore appear to be the more numerous. This is actually the 
case with a few species ; but he mentions several species in six 
Sonera, in which the females appear to bo much more numerous 
than the males. 71 The small size of the males in comparison with 
die females, which is sometimes carried to an extreme degree, and 
'heir widely different appearance, may account in some instances 
f °r their rarity in collections. 72 
Some of the lower Crustaceans are able to propagate their kind 
Sexually, and this will account for the extreme rarity of the males. 
With some other forms (as with Tanais and Cypris) there is reason 
*0 believe, as Fritz Muller informs me, that the male is much shortcr- 
Wd than the female, which, supposing the two sexes to be at first 
e qual in number, would explain the scarcity of the males. On the 
other hand this same naturalist has invariably taken, oil the shores 
of Brazil, far more males than females of the Diastylidie and of 
°ypridina; thus with a species in the latter genus, (53 specimens 
Ca 'tght the same day, iucluded 57 males ; but he suggests that this 
Preponderance may be due to some unknown difference in the habits 
r, f the two sexes. With one of the higher Brazilian oralis, namely 
a Gelasimus, Fritz Miillcr found the males to be more numerous 
d*aa the females. The reverse seems to be the case, according to 
large experience of Mr. C. Spence Bate, with six common British 
Ct obs, the names of which he has given me. 
On the Power of Natural Selection to regulate the pro- 
motional Numbers of the Sexes, and General Fertility . — 
Iff some peculiar cases, an excess in the number of one 
Sex over the other might be a great advantage to a 
species, as with the sterile females of social insects, or 
"ith those auimals in which more than one male is 
’ e< ]uisite to fertilise the female, as with certain cirri- 
P e des and perhaps certain fishes. An inequality be- 
tween the sexes in these cases might have been acquired 
through natural selection, but from their rarity they 
** e ed not here be further considered. In all ordinary 
j, ,l Another great authority in this class, Prof. Tliorell ofUpsala (‘On 
European Spiders,’ 1809-70, part i. p- 205) speaks as if female spiders were 
generally commoner than the males. 
> See, on this subject, Mr. Pickard-Cainbridge, as quoted in 1 Quarterly 
°Urnal of Science,’ 1868, p. 429. 
