W. K. BROOKS ON THE GENUS SALPA. 153 



are found in immense numbers and at considerable depths. In 1888 I 

 was invited by Marshall McDonald, the Superintendent of the IT. S. Fish 

 Commission, to make use of the opportunity for surface collecting which 

 was afforded by an expedition which was sent out to fish with hook and 

 line on the bottom and along the edge of the Gulf Stream. The fishing 

 commenced at the 500 fathom line, and every time the line was taken in 

 we found numbers of dogfish (Scyllium) on the hooks, even when the 

 water was considerably more than half a mile deep. 



Many genera of sharks, such as the houndfish (Mustelus) and the 

 dogfish (Scyllium), are known to feed upon the molluscs and Crustacea 

 and worms of the bottom, and the flat pavement-teeth of other genera 

 whose habits are less known show that their mode of life is the same. 

 Some of the bottom-feeding sharks (Cestracion for example) are the 

 oldest of living vertebrates. 



The mailed ganoids were undoubtedly derived from a shark-like 

 ancestor, and the structure of the oldest ones, such as perichthys, cocco- 

 steus and cephalaspis, shows that they were not very rapid swimmers. 

 They were, undoubtedly, bottom-feeders like the modern sturgeon, and 

 like many large and important families of modern teleosts, such as the 

 cod, the siluroids and the pleuronectidae. 



So far as we know the palaeozoic waters from fossils, there were no 

 active locomotor animals of large size to furnish prey for raptorial fishes, 

 and the existence at the present day of so many species and genera and 

 families of bottom-feeders, and the fact that the most archaic forms have 

 this habit, are all grounds for believing that the fishes are secondarily 

 adapted to a pelagic life, like the sea-birds and the cetacea. 



So far as amphioxus furnishes evidence, this bears in the same 

 direction, for its home is in the sand of the bottom. In fact it may 

 almost be called a subterranean animal, for when it is placed in an 

 aquarium it sinks into the sand at the bottom and disappears at once, 

 and it makes its way through the sand with great ease and rapidity. 



All the evidence shows that the primitive vertebrates lived upon or 

 near the bottom, and that the early steps in the evolution of the classes 

 of vertebrated animals were made at the bottom. 



As the result of this review we see that the evidence from palaeon- 

 tology, from embryology, and from the structure and habits of living- 

 animals all bears in the same direction, and shows that there are no 

 large or highly organized animals which have been pelagic through all 

 the stages of their evolution, and that, in this particular, the life-history 

 of Salpa is not exceptional, but typical. 



