ADDITIONS 177 



After the fishes had led up to the formation of the batrachian 

 reptiles and these to the ophidian reptiles, both of which have only one 

 auricle in their hearts, nature easily succeeded in giving a heart with a 

 double auricle to the other reptiles, which became divided into two sepa- . 

 rate branches ; subsequently she easily achieved the formation of a heart 

 with two ventricles in animals originating from both these branches. 



Thus among the reptiles which have a heart with a double auricle, 

 the chelonians appear to have given existence to the birds ; for, in 

 addition to their various unmistakable affinities, if I were to place 

 the head of a tortoise on the neck of certain birds, I should find hardly 

 any incongruity in the general appearance of the factitious animals ; in 

 the same way the saurians, especially the planicauds, such as croco- 

 diles, seem to have led to the existence of the amphibian mammals. 



If the chelonian branch has given rise to the birds, we may suppose 

 that the aquatic palmipeds, and especially the brevipens, such as the 

 penguins and king-penguins, have brought about the formation of the 

 monotremes. 



Lastly, if the saurian branch gave rise to the amphibian mammals, 

 it is highly probable that from this branch all the mammals have 

 taken their origin. 



I think the behef is justifiable, that the terrestrial mammals originated 

 from those aquatic mammals that we call amphibians. These were 

 divided into three branches by reason of the diversity arising in their 

 habits in the course of time ; one of these led to the cetaceans, another 

 to the ungulate mammals, and the third to the various known un- 

 guiculate mammals. 



Those amphibians indeed which preserved the habit of going on to 

 the beach became divided, owing to their different manner of feeding. 

 Some of them, being accustomed to browsing on grass, as for instance 

 the walruses and manatees, gradually led to the formation of the 

 ungulate mammals such as the pachyderms, ruminants, etc. ; the others 

 as, for instance, the seals, having acquired the habit of feeding ex- 

 clusively on fishes and marine animals, brought about the existence 

 of the unguiculate mammals through the medium of races which 

 as they diversified became altogether terrestrial. 



Those aquatic mammals, however, which acquired the habit of 

 never coming out of the water and of only coming to the surface to 

 breathe, probably gave rise to the various cetaceans with which we 

 are acquainted. The cetaceans have been greatly modified in organisa- 

 tion by having dwelt for so long a period exclusively in the sea ; hence 

 it is now very difficult to recognise whence they derive their origin. 



In consequence of the immense lapse of time during which these 

 animals have lived in the sea without ever using their hind-legs for 



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