If animals and plants have arrived at their present state by descent with modification from simpler forms, it ought to be possible by careful searching to trace the line of ancestry; and it is this fascinating but frequently futile pursuit which has dominated the minds of many of our ablest zoölogists for the last thirty years. To such an extent has this pedigree hunting been carried that there is scarcely a group of invertebrates from which the vertebrates have not been theoretically derived; and to-day one of the ablest of our physiologists is using his great powers in the attempt to trace the origin of the backboned animals from a spiderlike creature, and is exercising his ingenuity in a plausible but unconvincing effort to equate the organs of a king-crab with those of a lamprey. This appeal to comparative anatomy and the consequent neglect of living animals and their habits are no doubt partly due to the influence of Huxley, Darwin’s most brilliant follower and exponent. He had the engineer’s way of looking at the world, and his influence was paramount in many schools. The trend which biology has taken since Darwin’s time is also partly due to a fervent belief in the recapitulation theory, according to which an animal in developing from the egg passes through phases which resemble certain stages in the past history of the ancestors of the animal. For example, there is no doubt that both birds and mammals are descended from some fishlike animal that lived in the water and breathed by gills borne on slits in the gullet, and every bird and mammal passes through a stage in which these gill-slits are present, though their function is lost and they soon close up and disappear. In the hope, which has been but partially realized, that a knowledge of the stages through which an animal passes on its path from the ovum to the adult would throw light on the origin of the race, the attention of zoölogists has been largely concentrated on details of embryology, and a mass of facts has already been accumulated which threatens to overwhelm the worker.