652 PROK. HUXLEY ON THE [DcC. lA, 



of the purpose of classification defined above) exactly in so far as it 

 expresses tlie 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 develojjment 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 con- 

 sequence of the doctrine of evolution ; and its establishment con- 

 stitutes, I believe, the foundation of scientific taxonomy. 



If all the Mammalia are the results of a process of evolution ana- 

 logous to that which has taken place in the case of the Equidse, and 

 if they exhibit different degrees of that process, then a natural classi- 

 fication 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 raammalium " 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 deve- 

 lopment, 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 : — 



1. 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 hypoojstic ; 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 vertebree are but slightly, or not at all, 

 developed'. 



Dr. Albrecht ("Die Epiphyseu uud die Amphiomphalie der Saugethiei- 



