HUXLEY AND HIS WORK. 775 



and endowed with older names. The memoir at least gave an impulse 

 in the right direction — morphological as opposed to teleological — and 

 has incited to many elaborate investigations to the great advantage of 

 ornithology. 



IX. 



Much doubt had existed respecting the nature of the non-mammalian 

 ancestors of the mammals. It was supposed by some that they must 

 have been reptiles related to the Dinosauriaus, but the specialized 

 characteristics and high development of that type forbade the belief 

 that they were in the direct line of descent. Of course the birds 

 which agreed with the mammals in the possession of a quadrilocular 

 heart, complete circulation, and warm blood must even more positively 

 than the Dinosaurians be excluded from the line of descent. The 

 problem of what was the genealogy of the highest class of animals was 

 at last attacked by Huxley. In several memoirs, 1 published in 1876, 

 1879, 2 and 1880, 3 he examined the evidence and formulated his con- 

 clusions. Those conclusions were expressed in the following terms : 



" Our existing classifications have no place for [the] submammalian 

 stage of evolution (already indicated by Haeckel under the name of 

 Prbmammale). It would be separated from the Sauropsida by its two 

 condyles, and by the retention of the left as the principal aortic arch; 

 while it would probably be no less differentiated from the Amphibia by 

 the presence of an amnion and the absence of branchiae at any period 

 of life. I propose to term the representatives of this stage JSypotheria; 

 and I do not doubt that when we have a fuller knowledge of the ter- 

 restrial vertebrata of the later palaeozoic epochs, forms belonging to 

 this stage will be found among them. Now, if we take away from the 

 Hypotheria the amnion and the corpus callosum, and add the functional 

 branchiae — the existence of which in the ancestors of the mammalia is 

 as clearly indicated by their visceral arches and clefts as the existence 

 of complete clavicles in the ancestral Canidae is indicated by their ves- 

 tiges in the dog — the Hypotheria, thus reduced, at once take their place 

 among the Amphibia, for the presence of branchiae implies that of an 

 incompletely divided ventricle and of numerous aortic arches, such as 

 exist in the mammalian embryo, but are more or less completely sup- 

 pressed in the course of its development. 



"Thus I regard the amphibian type as the representative of the next 

 lower stage of vertebrate evolution; and it is extremely interesting to 



'On the evidences as to the origin of existing vertebrate animals. (Nature, Vol. 

 XIII, pp. 388, 389, 410-412, 429, 430, 467-469, 514-516; Vol. XIV, pp. 33, 34. 



2 On the characters of the pelvis in the mammalia, and the conclusions respecting 

 the origin of mammals which maybe based on them. (Proc. Royal Soc, Vol. XXVIII, 

 pp. 162, 163; Nature, Vol. XX, pp. 22-24.) 



3 On the application of the laws of evolution to the arrangement of the vertebrata, 

 and more particularly of the mammalia. (Proc. Zool. Soc, 1880, pp. 649-662 ; Nature, 

 Vol. XXIII, pp. 203, 204, 227-231.) 



