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Show 652 PROF. HUXLEY ON THE [Dec. H, of the purpose of classification defined above) exactly in so far expresses the relations of likeness and unlikeness enumerated under these heads. Hence, in attempting to classify the Mammalia, we must take into account not only their adult and embryogenetic characters, but their morphological relations, in so far as the several groups represent different stages of evolution. And thus, just as the persistent antagonism of Cuvier and his school to the essence of Lamarck's teachings (imperfect and objectionable as these often were in their accidents) turns out to have been a reactionary mistake, so Cuvier's no less definite repudiation of Bonnet's "echelle des etres" must be regarded as another unfortunate effort to oppose the development of just biological conceptions. For though no one will pretend to defend Bonnet's "echelle" at the present day, the existence of a " scala animantium" is a necessary consequence of the doctrine of evolution ; and its establishment constitutes, I believe, the foundation of scientific taxonomy. If all the Mammalia are the results of a process of evolution analogous to that which has taken place in the case of the Equidse, and if they exhibit different degrees of that process, then a natural classification will arrange them, in the first instance, according to the place which they occupy in the scale of evolution of the mammalian type, or the particular rung of the "scala mammalium" on which they stand. The determination of the position thus occupied by any group may, I think, be effected by the deductive application of the laws of evolution. That is to say, those groups which approach the non-mammalian Vertebrata most closely, present least inequality of development, least suppression and least coalescence of the fundamental parts of the type, must belong to earlier stages of evolution; while those which exhibit the contrary characters must appertain to later stages. Judged from this point of view, there can be no doubt that the Monotremes embody that type of structure which constitutes the earliest stage of mammalian organization :- I. The mammary glands are devoid of teats; and thus the essential feature of the mammal could hardly be presented under a simpler form. 2. There is a complete and deep cloaca, as in Vertebrata lower in the scale. 3. The openings of the ureters are hypocystic ; that is to say, they open not into the bladder of these animals, but behind it, into the dorsal wall of the genito-urinary passage. As this answers to the neck of the allantois, the ureters of the Monotremes retain their primitive embryonic position. 4. There is no vagina apart from the genito-urinary passage; and the oviducts are not differentiated into distinct uterine and Fallopian regions. 5. The penis and the clitoris are attached to the ventral wall of the cloaca. 6. The epiphyses of the vertebrce are but slightly, or not at all, developed1. 1 Dr. Albrecht (" Die Epiphyses und die Amphiomphalie der Siiugethier- |