184 A. A. W. TniBKECIIT. 



in the vertebi-ate kingdom. Wliat is preserved to us in the 

 I'ecent fauna inhabiting tliis planet is only the faintest echo of 

 the multitudinous and protean changes that have, during the 

 course of time, succeeded one another. And it has been our 

 mistake to attempt to co-ordinate the present stages of de- 

 velopment with each other iti such a sense that they were 

 expected to represent, in lineary arrangement, the successive 

 evolutionary stages of those foetal envelopes. 



How false the conclusions may be to which this method may 

 lead us is best exemplified by what is at present often taught 

 concerning, e.g., placentation, a phenomenon in which the 

 outer larval layer, the trophoblast, plays such a prominent 

 part. You will find in the text-books that this was started 

 l)y what is called the diffuse placentation as it is at present 

 met with in many ungulates, in the lemurs, and in certain 

 Edentates. It is my conviction that this doctrine is utterly 

 false. The diifuse placentation is no placentation at all ! The 

 horse and the lemur are, by birthright, aplacental animals, 

 much more so than marsupials, such as Perameles aud 

 Dasynrus, which have hitherto ranked among the Mammalia 

 aplacentalia. And still, by careful comparison of various 

 data, we can soon discover that the diffuse placentation, and 

 that variety of it which is styled the polycotyledouary, far 

 from being arcliaic or primitive, is, on the contrary, very largely 

 a secondai-y modification. Among the living Carnivora we 

 find several intermediate stages, not in the sense that these 

 have been phylogenetic transitions, but in that wider sense 

 that these Carnivora demonstrate the possibility how more 

 intricate placentary structui'es may finally have led up to a 

 diffuse placentation, as that of the horse and the pig, conse- 

 quent upon an increase in the area of surface contact between 

 mother and foetus. What was originally a small surface of 

 intense interchange (Procavia) has then gradually become an 

 extended surface, along which two epithelial layers, one 

 maternal and one foetal, between the blood of the mother and 

 the blood of the embr3^o, offered no impediment for a suflBcient 

 interchange of nutritive matter and of oxygen. 



