Observations upon Tropical Fishes and Inferences from their Adaptive Coloration.

tribution and more or less evanescent in continuity, to the streams that flowed constantly in one direction and eventually into an unnatural habitat, the sea. They needed a more direct and quicker embryonic development, requiring, it would seem, the abandonment of the nauplius stage. It may therefore well be that the trilobites retaining the nauplius stage did not give rise to these stocks, as is sometimes assumed. We may have to look for this ancestral stock in one still more primitive, and the Protocaris-Apus line of branchiopods suggests itself, but whatever the stock, it would seem to have permanently invaded the rivers of the land either in Proterozoic time, or that postulated intermediate stage in the earth's history previous to the Cambrian but of which we have not a trace of direct evidence, the Lipalian time of Walcott.