=Ontogeny. Phylogeny.=--In the light of the facts of evolution, heredity takes quite a new aspect when removed from the old biblical idea of the independent creation of species. _Haeckel_ launched into the scientific world, under the name of "fundamental biogenetic law," a theory which, without having the right to the title of an immutable dogma, explains the facts in a general way, and gives us a guiding line along the phylogenetic history of living beings. "_Ontogeny_," that is the history of the embryological development of each individual, always consists in a summary and fragmentary repetition of _phylogeny_, or the history of the ancestors of the species to which the individual belongs. This signifies that, as embryos, we repeat in an abridged form the series of types or morphological stages through which has passed the series of our animal ancestors, from the primitive cell to man. In reality this is only true in a relative way, for a considerable part of the ancestral engraphias of the embryo has disappeared without leaving any trace; also many embryos, especially those which have special conditions of existence outside the body of their mother, have acquired special complex organs and corresponding functions. Thus, the caterpillars of butterflies with their specific and generic peculiarities, hairs, horns, etc., furnish many examples of secondary acquired characters which have nothing in common with the worm, which is the ancestral type of the butterfly represented by the embryonic period when it is a caterpillar. However, many undoubted vestiges of the ancestral history are found in the embryos at different periods of their development. It is certain that insects descended from worms, and there is no doubt that the larvæ of insects, which are almost worms, represent the ontogenetic repetition of the phylogeny of insects. It is also certain that whales, although they have whalebone instead of teeth, have descended from cetacea provided with teeth, which in their turn descended from terrestrial mammals. But we find in the embryo whale a complete denture which is of no use to it, and which disappears in the course of the embryonic period. This denture is nothing else than a phylogenetic incident in the ontogeny of the whale. In the fins of cetacea, as in the four limbs of other mammals, we find the same bones, which are derived from the bones of the wings and legs of their bird ancestors. In birds, the same bones are the phylogenetic derivatives of the limbs of reptiles. All these facts demonstrate with certainty the descent of animal forms, a descent which we can follow in all its details. In certain ants whose bodies show their close relationship with a slave-keeping group, but which have become the parasitic hosts of other ants, we find not only the arched mandibles, shaped for rape, but the undoubted rudiments of the slave instinct, although this instinct has, perhaps, not been exercised by them for thousands of years.