The difficulties in the way of the study of the behaviour of fishes during the critical period of mate-hunting are many and obvious. Something may be inferred from the nature of the secondary sexual characters which they exhibit, and more definite information can be obtained from such species as can be kept in aquariums. From these two sources enough has been gleaned to show that these cold-blooded creatures, in many cases, exhibit the same emotions and the same means for their fulfilment as the higher vertebrates. And it is significant that wherever anything approaching what may be called “Courtship” obtains, the males commonly exhibit secondary sexual characters, whether in the form of ornament or of armature; while among species which consort in shoals during the breeding season no such distinctions are present. The ova and milt are shed and fertilization takes place as they escape. Comment is frequently made in works on Natural History on the fact that among fishes the males are commonly smaller, often conspicuously so, than the females. Among mammals the males are the larger; but among birds this is by no means always the case. It is somewhat surprising to find this discrepancy among the birds of prey, where, as in the case of the Sparrow-hawk, the male is little more than half the size of his mate; commonly, however, there is little or no difference. Among the fishes the differences are often much more marked, as for example in the Conger-eel, wherein the male never exceeds a length of two feet six inches or a weight of one pound; females, on the other hand, may exceed eight feet in length and attain a weight of one hundred and twenty-eight pounds, though such giantesses are rare, but specimens of fifty pounds and upwards are frequently met with. The explanation of this may lie in the fact that among fishes it is no uncommon thing to and males becoming sexually mature long before they have attained their full stature. With the Salmon, for instance, ripe spermatozoa have been found in individuals of not more than a few inches in length, and in this species also the male is the smaller. Ova take longer to attain maturity, for in addition to the germ-plasm they must be provided with a more or less extensive amount of food-material in the shape of yolk. The formation of this is inhibited until the demands on the system for the building up of the body have begun at least to lessen.