92 ON CLASSIFICATION. 



two divisions of the Implacentalia are separated by characters 

 of fully as great importance as those which distinguish the 

 Placentalia and Implacentalia. 



But whether the Omithodeljphia and the BidelpJiia are re- 

 garded (as I believe they ought to be) as two of the three 

 primary separate " sub-classes " of the class Mammalia, or 

 whether they are looked upon only as subdivisions of the Im- 

 placentalia, there is no doubt that they are, and will remain, dis- 

 tinct natural assemblages, the subdivisions of which present no 

 very great difficulties. 



It is otherwise with the sub-class Monoclelpliia— which con- 

 tains at least a dozen orders, the arrangement of which into 

 groups, not only in detail, but in principle, is, and long has been, 

 a subject of much difficulty, and consequently of controversy. 



Sir Everard Home* is commonly quoted as the originator 

 of one of the two systems of classification in vogue at the 

 present day ; but his vague statements and confused notions 

 respecting the varying characters of the placenta of the 

 Monodeljihia hardly entitle him to that honour, which, in my 

 opinion, belongs rather to that eminent man, Karl Ernst von 

 Baer, of whom it can be truly said that he has touched no sub- 

 ject without throwing a flood of light upon it. Towards the 

 end of his famous essay, " Untersuchungen iiber die Gefas- 

 verbindung zwischen Mutter und Frucht," published in 1828, 

 the following passage occurs : — 



" In the first place, I have taken pains to show that the ova 

 of mammals are only variations of a single type ; and if we 

 except the ova of the Marsupials, concerning which I can 

 form no judgment, all consist of the same parts ; all have a 

 placenta; and, in all, some portion of the chorion is smooth. 

 The foetal placenta consists everywhere of the same elements, 

 but offers the most remarkable differences in its external dis- 

 position. It is either — 



1. Merely applied to the maternal placenta, and 



(a) continuous and zone-like. First form. 



(b) divided into many parts. Second form. 



* " Comparative Anatomy," vol. iii. 



