DISCUSSION ON ORIGIN OF MAMMALS. 75 



of snakes, the feet of ungulates, and the absence of teeth in Hving 

 birds, there was no sign of a more complete development in the 

 embryonic state. As to the earliest stages of mammalian embryos, 

 which differed considerably in the various orders, but little import- 

 ance can be attached to them, when we remember the fact that 

 within the limit of a single genus, viz. Pcripatiis, a greater amount 

 of variation is found in the earliest embryos than in the whole 

 Mammalian class. 



Finally on examining the relationship of living and extinct 

 animals and endeavouring to classify them, and remembering that 

 all the great classes of the animal kingdom which could be pre- 

 served as fossils had already made their appearance in the lowest 

 Paleozoic rocks, one could not but be struck by the fact that 

 there was much to be said for the view that the greater part of 

 evolutionary change had already taken place in precambrian times 

 before the fossiliferous period. If this view were correct, — and the 

 possibility of it should be borne in mind, — the main part of 

 the evolution of organisms must have taken place under totally 

 different conditions to those now existing, and must remain for 

 ever unknown to us. We should then have to look upon fossils in 

 the main as being rather collaterally than ancestrally connected 

 with living forms, and evolution during the fossiliferous period as 

 having consisted mainly of a process of extinction of the less fit 

 varieties than of the formation of important new types. 



Professor HUBRECHT, in closing the discussion, said, in reply 

 to Mr Sedgwick, that the value of embryology was in many cases 

 destructive, not constructive. Its evidence was of value as often 

 prohibiting certain lines of speculation. He differed from his 

 great teacher. Professor Haeckel's present views, which he thought 

 untenable, since Hill has shown that in Perameles, a genus of 

 Australian Marsupials, a distinct placenta is found, which is de- 

 ciduous. What Caldwell and Semon have shown to exist in another 

 Marsupial, Phascolarctos, seems rather to be a phase of retrogressive 

 than of incipient placentation. He predicted that one great battle- 

 field in the future of this controversy would be over the question 

 whether mammals other than Monotremes had descended from 

 oviparous ancestors. 



Professor Newton proposed a vote of thanks to Professor 

 Hubrecht for his conduct in the chair. 



