222 L. Agassiz on the Natural Relations between 



theii' wings, in the more scale-like form of their feathers, 

 and the greater number of eggs they lay, and the less care 

 they take of their young, which are hatched in a state of 

 development in which they are already prepared to provide 

 for their own food. The same is the case with the Gallina- 

 ceous and the Wading birds, which, though more advanced 

 in many respects, are still inferior to the climbing and Pas- 

 serine 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 fingers, and the Gallinaceous birds, as well as the 

 Ostriches, having 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 

 important 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. 



4. Mammals. — The class of Mammalia is not only the 

 most diversified in the forms of its members, but also in 

 the diversity of their mode of life ; nevertheless, this di- 

 versity is connected by the most intimate relations of struc- 

 ture. The Whales are as much Mammalian by their in- 

 ternal organization, as the most exclusively terrestrial quad- 

 rupeds. 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 con- 

 sidered as entirely distinct from the true Cetaceans ; and 

 those Cetaceans, at the same time that they are so exclu- 

 sively 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 fin, 



