20 
SEXUAL SELECTION. 
Pap.t II. 
with the dull-coloured gobies/^ in wliich the sexes are 
not known to differ in colour, and likewise with the 
sticklebacks (Gasterosteus), in which the males become 
brilliantly coloured during the spawning-season. The 
male of the smooth-tailed stickleback (G. leiurus) per- 
forms during a long time the duties of a nurse with 
exemplary care and vigilance, and is continually 
employed in gently leading back the young to the 
nest when they stray too far. He courageously 
drives away all enemies, including the females of his 
own species. It would indeed be no small relief to the 
male if tlie female, after depositing her eggs, were 
immediately devoured by some enemy, for he is forced 
incessantly to drive her from the nest."^ 
The males of certain other fishes inhabiting South 
America and Ceylon, and belonging to two distinct 
orders, have the extraordinary habit of hatching the 
eggs laid by the females within their mouths or branchial 
cavities.^'^ With the Amazonian species which follow 
this habit, the males, as I am informed by the kindness 
of Professor Agassiz, ‘Giot only are generally brighter 
than the females, but the difference is greater at 
the spawning-season than at any other time.” The 
species of Geopliagus act in the same manner ; and in 
this genus, a conspicuous protuberance becomes deve- 
loped on the forehead of the males during the breeding- 
season. With the various species of Chromids, as Pro- 
fessor x^gassiz likewise informs me, sexual differences 
32 Cuvier, ‘ Kegne Animal/ vol. ii. 1829, p. 212. 
33 See Mr. Warington’s most interesting description of the habits of 
the Gasterodeus leiurus^ in ‘ Annals and Mag. of Xat. Hist.’ Xovemb:.'r, 
1855. 
3-^ Prof. Wyman, in ‘ Proc. Boston Soc. of Nat. Hist.’ Sept. 15, 1857. 
Also W. Turner, in ‘ Journal of Anatomy and Phys.’ Nov. 1, I860, 
p. 78. Dr. Gunther has likewise described other cases. 
