20 
SEXUAL SELECTION. 
Pap.t II. 
with the dull-coloured gobies , 32 in which 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 hack 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 the female, after depositing her eggs, were 
immediately devoured by some enemy, for he is forced 
incessantly to dx - ive her from the nest . 33 
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 . 84 With the Amazonian species which follow 
this habit, the males, as I am informed by the kindness 
of Professor Agassiz, “not only are generally brighter 
“ than the females, hut the difference is greater at 
“ the spawning-season than at any other time.” The 
species of Geophagus act in the same manner ; and in 
this genus, a conspicuous protuberance becomes deve- 
loped ou the forehead of the males during the breeding- 
season. W ith the various species of Chrom ids, as Pro- 
lessor Agassiz likewise informs me, sexual differences 
Cuvier, ‘ Eegne Atiimal,’ vol. ii. 1829, p. 212. 
:!;i See Mr. Warington’s most interesting description of the habits of 
the Gasterosteus leiurus, in ‘Annals anil Mag. of Nat. Hist.’ November, 
1855. 
84 Prof. Wyman, in ‘Proc. Boston Soe. of Nat. Hist.’ Sept. 15, 1857. 
Also W. Turner, in ‘Journal of Anatomy and Pliys.’ Nov. 1, 1866, 
p. 78. Dr. Gunther has likewise described other cases. 
