144 W. K. BROOKS. 



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 particu- 

 lar, the life-history of Salpa is not exceptional, but typical. 



In its descent from an inhabitant of the bottom and in its 

 secondary adaptation to a pelagic life, its history resembles that of 

 all the highly organized pelagic animals. 



Embryology also gives us good ground for believing that Salpa 

 follows the analogy of all the metazoa in its still more remote 

 descent from a small and simple pelagic ancestor, and there is 

 good ground for believing that the earliest metazoa were all 

 pelagic, and that they were represented at a very early period in 

 the history of life by floating or swimming animals of minute size 

 and simple structure. We may see in the free larval forms of 

 many marine metazoa, such as the tornaria of balanoglossus, the 

 swimming echinoderra larva, the ascidian tadpole, the floating 

 ciliated larvae of annelids, brachiopods and molluscs, in the coelen- 

 terate planula, and, as I believe, in the crustacean nauplius, traces 

 of this primitive mode of life ; often obscured or complicated 

 by more recent adaptation and sometimes almost obliterated by 

 secondary changes. 



When this fact is seen in all its bearings and its full signficance 

 is grasped, it is certainly one of the most noteworthy and instruc- 

 tive features of the history of evolution. 



