In man and other vertebrates, hermaphrodism is always abnormal. In man it is extremely rare and nearly always very incomplete, being usually limited to the external or correlative characters. =Heredity.=--It results from what we have said that every living being reproduces, more or less identically, in its specific characters, the whole life of its parents and less remote ancestors, and constitutes the continuation of life from a minute part of their bodies. Each individual life thus repeats an entire cycle of development called _ontogeny_, which is peculiar to all individuals of the species. Here we must mention three fundamental points: (1). In its principal characters, each individual is the copy of its parents or direct ancestors, with correlative sexual peculiarities which we have mentioned, and with individual variations due to the combinations of varieties by conjugation, and the alternating or unequal ecphorias of hereditary characters; that is to say paternal or maternal hereditary engrams. (2). No individual is absolutely identical with another. (3). On the average, each individual resembles more especially its direct ancestry and its parents, and differs more markedly from its parentage the more this is remote. We shall see later on that the ancestral relationship of the different groups, species and varieties of animals has been fairly well fixed, and we may say that the third of the laws stated above is equally true in a wider sense. In fact the species and varieties of animals which are near related resemble each other, while the _genera_, families and classes are more dissimilar as their relationship is more remote. We employ here the terms resemblance, homology and difference in their profound and general sense. Certain purely external resemblances, due to phenomena of convergence, must not be considered as homologies in the sense of hereditary relationship. Thus, in the language of natural history we do not say that a bat resembles a bird, nor that a whale resembles a fish, for here the resemblances are due simply to aërial or aquatic life which produces the effects of convergence, while the internal structure shows them to be quite dissimilar organisms. Although it swims in the sea the whale is a mammal; its fins at first sight resemble those of a fish, but they are really the homologues of the four limbs of other mammals and contain the corresponding bones. In man, we see that brothers and sisters resemble each other in a general way, but that each one is dissimilar in some respects from the others. If we compare different families with many children we find that brothers and sisters resemble each other the more their parents are alike and come from a uniform ancestry which has undergone little crossing, while the crossing of different races and human varieties results in the production of individuals which differ from each other considerably, even when they come from the same couple.