Xv BREEDING 413 
vantages attendant on the sexual relations of Fishes, involving a 
considerable waste of the sex-cells, while in many Fishes it no 
doubt helps to compensate for any subsequent mortality among 
the larvae, which may result from an uncertain and precarious 
food supply and from the attacks of enemies. Whenever internal 
fertilisation is the rule, or when, as in nest-building and mar- 
supial Fishes, the propinquity of the sexes in the breeding season 
ensures the fertilisation of a larger proportion of the eggs and the 
protection of the young, the number of eggs produced is small. 
The male. sex-cells or spermatozoa are essentially similar to 
those of other Vertebrates, although in different Fishes they may 
vary in such details as length, and the shape and size of the 
head, which may be rod-like and wavy, elliptical or globular. 
As a rule, in Fishes females are more numerous than males, 
and generally they are larger, but to both statements there are 
notable exceptions. The relations of the sexes in the breeding 
season are usually very promiscuous, especially in those Teleosts 
which discharge their sex-cells while swimming together in 
shoals. A female may, however, consort with several males 
(polyandry), or a male with several females (polygamy); or, as in 
some of the nest-building Fishes (e.g. Gastrosteus), there are not 
wanting examples of the pairing of one male with one female 
(monogamy). 
Fishes often migrate at the commencement of the breeding- 
season to localities most suitable for the deposition of the eggs. 
Many marine species seek banks or shallower water near 
the shore, and some, like the Salmon and the Sturgeon, are 
anadromous, and ascend rivers for long distances to deposit their 
spawn. 
In all Fishes except the Elasmobranchs and a few Teleosts 
the fertilisation of the eggs takes place in the water after their 
extrusion, the male depositing its seminal fluid over the eggs or 
in their neighbourhood. The waste of the sex-cells is often, no 
doubt, very considerable, especially when the eggs are adhesive 
and fixed, and the seminal fluid is lable to drift at the mercy of 
tides and currents. With pelagic ova the waste is perhaps not 
so great, inasmuch as the eggs as well as the spermatozoa 
would probably drift at the same rate and in the same direction. 
Liability to waste must also be greatly diminished in many 
Fishes by their habit of living in shoals, or of congregating 
