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The Descent of Man and Selection in Relation to Sex

Secondary Sexual Characters Of Fishes, Amphibians,
And Reptiles
FISHES: Courtship and battles of the males--Larger size of the females-- Males, bright
colours and ornamental appendages; other strange characters-- Colours and appendages
acquired by the males during the breeding-season alone--Fishes with both sexes
brilliantly coloured--Protective colours--The less conspicuous colours of the female
cannot be accounted for on the principle of protection--Male fishes building nests, and
taking charge of the ova and young.
AMPHIBIANS: Differences in structure and colour between the sexes--Vocal organs.
REPTILES: Chelonians--Crocodiles--Snakes, colours in some cases protective--Lizards,
battles of--Ornamental appendages--Strange differences in structure between the sexes--
Colours--Sexual differences almost as great as with birds.
We have now arrived at the great sub-kingdom of the Vertebrata, and will commence
with the lowest class, that of fishes. The males of Plagiostomous fishes (sharks, rays) and
of Chimaeroid fishes are provided with claspers which serve to retain the female, like the
various structures possessed by many of the lower animals. Besides the claspers, the
males of many rays have clusters of strong sharp spines on their heads, and several rows
along "the upper outer surface of their pectoral fins." These are present in the males of
some species, which have other parts of their bodies smooth. They are only temporarily
developed during the breeding-season; and Dr. Gunther suspects that they are brought
into action as prehensile organs by the doubling inwards and downwards of the two sides
of the body. It is a remarkable fact that the females and not the males of some species, as
of Raia clavata, have their backs studded with large hook-formed spines. (1. Yarrell's
'Hist. of British Fishes,' vol. ii. 1836, pp 417, 425, 436. Dr. Gunther informs me that the
spines in R. clavata are peculiar to the female.)
The males alone of the capelin (Mallotus villosus, one of Salmonidae), are provided with
a ridge of closely-set, brush-like scales, by the aid of which two males, one on each side,
hold the female, whilst she runs with great swiftness on the sandy beach, and there
deposits her spawn. (2. The 'American Naturalist,' April 1871, p. 119.) The widely
distinct Monacanthus scopas presents a somewhat analogous structure. The male, as Dr.
Gunther informs me, has a cluster of stiff, straight spines, like those of a comb, on the
sides of the tail; and these in a specimen six inches long were nearly one and a half inches
in length; the female has in the same place a cluster of bristles, which may be compared
with those of a tooth-brush. In another species, M. peronii, the male has a brush like that
possessed by the female of the last species, whilst the sides of the tail in the female are
smooth. In some other species of the same genus the tail can be perceived to be a little
roughened in the male and perfectly smooth in the female; and lastly in others, both sexes
have smooth sides.
The males of many fish fight for the possession of the females. Thus the male stickleback
(Gasterosteus leiurus) has been described as "mad with delight," when the female comes
out of her hiding-place and surveys the nest which he has made for her. "He darts round
her in every direction, then to his accumulated materials for the nest, then back again in
 
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