176 M. Agassiz on the Relations between Animals 



pared to provide for their own food. The same is the case with 

 the Galhnaceous and the Wading Birds, which, though more ad- 

 vanced in many respects, are still inferior to the climbing and 

 Passerine Birds in this respect, having a heavier flight, if they fly 

 at all, and living a more terrestrial, and even aquatic life ; the 

 Wading Birds coming nearer in this respect to those with palmate 

 Angers, and the Gallinaceous Birds, as well as the Ostriches, ha- 

 ving a more terrestrial mode of life; whilst the Passerine Birds rank 

 higher in all these respects, feed their young, and take care of 

 them for a longer time, and live almost exclusively an aerial life, 

 few of them having aquatic habits, and those being in their 

 respective families by their form as well as by their mode of life, 

 decidedly inferior to their loftier relations. 



The classification of Birds as a whole is still so imperfect, 

 though their minor groups are well understood, that many im- 

 portant relations in these respects must necessarily be more or 

 less concealed as long as their primary divisions are not better 

 known; so that we may expect many interesting hints from 

 further investigations in this view. 



The class of Mammalia is not only the most varied in the 

 forms of its members, but also in the diversity of their mode of 

 life; nevertheless this diversity is connected by the most inti- 

 mate relations of structm'e. The Whales are as much mamma- 

 lian by their internal organization as the most exclusively ter- 

 restrial quadrupeds. True Cetaceans constitute a natural family, 

 all the members of which are exclusively marine, and no one 

 of them even fluviatile — for the Sirenidse must be considered as 

 entirely distinct from true Cetaceans ; and these Cetaceans, at 

 the same time that they are so exclusively marine, are also the 

 lowest type of Mammalia, not only from the imperfection of their 

 extremities, of which there is only one anterior pair, and from the 

 want of hind-legs, but also from the extraordinary development 

 and bulk of their muscular tail, and the development of a caudal 

 fln, and sometimes even a fin-like fold of the skin upon the back. 

 If it can be shown that the Sirenidae are an aquatic type of a 

 larger group embracing Pachyderms, the direct relation of their 

 structure and mode of life will be at once obvious, since Sirenidae 

 are either marine or fluviatile, while true Pachyderms are tez'res- 

 trial : and should we not be justified in considering the subaquatic 

 Hippopotamus as inferior to its more terrestrial relatives of the 

 genera Rhinoceros, Elephant and Horse ? Are we not to consider 

 the Ornithorhynchus, with its palmate hind-legs and spur, as in- 

 ferior to Echidna ? Are not the palmate llodentia inferior to the 

 terrestrial and arboreal types ? Are not the aquatic Shrews in- 

 ferior to the arboreal Insectivora 't All these secondary questions 

 will receive, in future, due attention, and will no doubt be satis- 



