GENERAL DISCUSSION. 9 
affected by longer and longer intervals, the length of these last being 
dependent on the duration of the exposure of the species to the lower 
temperature and minor factors. In this more or less lengthened increase 
of time required for development of eggs or growth of young appears 
an effective agent in isolation and in differentiation of the isolated species. 
What the effect of the retardation may be in regard to a tendency to ovo- 
viviparity is problematical. 
Some of the deep sea fishes evidently, as suggested by Agassiz, have 
been vertically derived from pelagic Berycoids, Scopelvids, etc. ; others, as 
is shown with tolerable conclusiveness in the collection before us, are 
descendants of forms living on the bottom in shoal waters, forms that have 
gradually traversed the slopes from the shores to the abysses. The larval 
fishes secured by the tow-net at intermediate depths are mostly young of 
pelagic species and young deep sea Scopeloids and others descended from 
a pelagic ancestry. 
Reasons for concluding that many of the less powerful swimmers among 
the bottom fishes have worked their way down, slid down as might be said, 
are seen on comparison of shoal and deep water forms in such cases as those 
of the Oncocephali (JJalthe, Cuv.) and the Halieutoids, of the Lophioids and 
the deep sea Pediculates like Ceratias and allies, of the Liparids of the shoals 
and the deep sea Discoboles, and of the shoal water Pleuronectoids and their 
abyssal relatives. Similar evidence is seen in the Ratidx, the Zoarcide, the 
Mureenide, the Myxinide, and others. 
It was expected that by means of the tow-net near the surface and at 
various distances below it, something might be learned concerning the young 
of species frequenting the great depths. Such expectations have not been 
realized. Numerous young fishes were obtained in the net, but they are 
identified with pelagic Berycoids, Scopeloids, and others of the intermediate 
upper waters, and in greater part with the shoal water fishes, Sebastoids, 
Percoids, Pomacentroids, Scomberesocoids, Squamipinnes, Plectognaths, 
Pleuronectoids, Murzenoids, ete., and together they give no light on the 
breeding habits of bathybial species. This experience is similar to that of 
the “Challenger,” which also by towing secured a fine lot of specimens of 
pelagic species, but among them few or none of those of the bottom. Among 
the deep sea specimens at hand there are small ones, more often apparently 
taken in depths somewhat less than those given for the large individuals of 
the same species. In these depths the temperatures were commonly some- 
