In the case of these older and lower vertebrates that lived in the water, the embryonic development had the palingenetic character in a still higher degree, as is the case in most of the fishes and amphibia of the present day. The familiar tadpole and the larva of the salamander or the frog still preserve the structure of their fish-ancestors in the first part of their life in the water; they resemble them, likewise, in their habits of life, in breathing by gills, in the action of their sense-organs, and in other psychic organs. Then, when the interesting metamorphosis of the swimming tadpole takes place, and when it adapts itself to a land-life, the fish-like body changes into that of a four-footed, crawling amphibium; instead of the gill-breathing in the water comes an exclusive breathing of the atmosphere by means of lungs, and, with the changed habits of life, even the psychic apparatus, the nervous system, and the sense-organs reach a higher degree of construction. If we could completely follow the psychogeny of the tadpole from beginning to end, we should be able to apply the biogenetic law in many ways to its psychic evolution. For it develops in direct communication with the changing conditions of the outer world, and so must quickly adapt its sensation and movement to these. The swimming tadpole has not only the structure but the habits of life of a fish, and only acquires those of a frog in its metamorphosis. It is different with man and all the other amniotes; their embryo is entirely withdrawn from the direct influence of the outer world, and cut off from any reciprocal action therewith, by enclosure in its protective membranes. Besides, the special care of the young on the part of the amniotes gives their embryo much more favorable conditions for the cenogenetic abbreviation of the palingenetic evolution. There is, in the first place, the excellent arrangement for the nourishment of the embryo; in the reptiles, birds, and monotremes (the oviparous mammals) it is effected by the great yellow nutritive yelk, which is associated with the egg; in the rest of the mammals (the marsupials and placentals) it is effected by the mother's blood, which is conducted to the foetus by the blood-vessels of the yelk-sac and the allantois. In the case of the most highly developed placentals this elaborate nutritive arrangement has reached the highest degree of perfection by the construction of a placenta; hence in these classes the embryo is fully developed before birth. But its soul remains during all this time in a state of embryonic slumber, a state of repose which Preyer has justly compared to the hibernation of animals. We have a similar long sleep in the chrysalis stage of those insects which undergo a complete metamorphosis--butterflies, bees, flies, beetles, and so forth. This sleep of the pupa, during which the most important formations of organs and tissues take place, is the more interesting from the fact