We know that with the exception of the marsupials (kangaroos, sarrigue), and the monotremata (ornithorhynchus, echidna), all mammals have a _placenta_, an organ essentially composed of a network of blood-vessels, which unites the mother to the fœtus, and serves for the nutrition of the latter. With the ruminants, the edentata, and the cetacea, the placenta is _simple_ and _diffuse_, that is to say, the tufts of the blood-vessels are developed upon the entire surface of the fœtal envelope, and are in direct communication with the inner surface of the uterus. In the rest of the mammals the placenta is _double_; half being derived from the mother, and half from the fœtus, or rather its external envelope. A special membrane called the _Decidua_ covers the interior of the uterus, and unites the placentæ. Haeckel, correctly attaching great importance to these anatomical differences, divides mammals into two great groups: the _indeciduata_, which have no decidua, and the _deciduata_, which possess it. Among the latter the placenta can surround the mammalian ovum like a girdle (_zonoplacentalia_), or form a kind of circular disc more or less developed (_discoplacentalia_). Man, apes, bats, insectivora, and rodents, present the latter arrangement, and thus form a natural group to which no _zonoplacential_, and, of course, no _indeciduate_ mammals can be admitted. Haeckel, without the least hesitation, adds his _prosimiæ_ to the groups which I have just enumerated, that is to say, he attributes to them a decidua and a discoidal placenta. Now the anatomical investigations of MM. Alphonse Milne Edwards and Grandidier upon the animals brought by the latter from Madagascar place it beyond all doubt that the prosimiæ of Haeckel have no decidua and a diffuse placenta. They are _indeciduata_. Far from any possibility of their being the ancestors of the apes, according to the principle laid down by Haeckel himself, they cannot even be regarded as the ancestors of the zonoplacential mammals, the carnivora for instance, and ought to be connected with the pachydermata, the edentata and the cetacea. Darwin and Haeckel will, perhaps reply that when they made their genealogies, the embryogenesis of the prosimiæ was not known. But why then represent them as one of the intermediate links to which they attach so much importance? Their process is always the same, considering the _unknown_ as a _proof_ in favour of their theory. VI. The necessity, which I think has been clearly proved, of seeking elsewhere than among the prosimiæ for the link which is required between the marsupials and the apes, would not invalidate the relationship between the latter and man. There are, however, other facts which are irreconcilable with the theory.