254 REPORT OF NATIONAL MUSEUM, 1892. 



of others, because their fossilizable parts were less indestructible in 

 some cases and the conditions necessary to their fossilization were less 

 favorable in others. Furthermore, they show that while the preserva- 

 tion of the remains of some animals was a natural result of the condi- 

 tions under which they lived, that of the remains of other auimals and 

 of plants was in all eases the result of accidental or unusual conditions. 



The following- brief 'review of the animal and vegetable kingdoms is 

 presented for the purpose of further applying the general statements 

 that have just been made to the subject of this essay and of com- 

 paring our presumably obtainable knowledge of extinct animal and 

 vegetable forms with our more definite knowledge of those which now 

 exist. The legitimate methods of this comparison have to some extent 

 just been indicated and they will be further shown on subsequent pages. 

 In the following remarks it will be necessary also to make some com- 

 parisons of the now living land and aquatic animals, respectively, with 

 those which lived in past geological epochs. 



If such a comparison could be made of all living with all extinct 

 animals, the proportion of now living land animals would doubtless be 

 shown to be much greater as a whole than it was during past geological 

 time, because in the earlier get >logieal periods there were probably no land 

 animals in existence, and their proportional numbers have since grad- 

 ually increased. In discussing certain of the higher classes, however, 

 mammals and birds for example, I assume that the proportion of 

 extinct aquatic to land denizens was not far from the same that it now 

 is, because the latter animals lived only in later geological time, dur- 

 ing which time their general conditions of life have probably suffered 

 comparatively little essential change 



Vertebrata. — Excluding si tine of the lowest and also some doubtful 

 or exceptional forms, all vertebrates possess either Avell developed teeth 

 or a bony skeleton, and much the greater part of them possess both. 

 Under favorable conditions the fossilization of these animal substances is 

 complete. Therefore, having fallen under such conditions, almost any 

 vertebrate animal which existed in former geological time is likely to have 

 left lossil remains. The epidermal structures, such as horns, hoofs, 

 feathers, etc., which cover either the whole or portions of the bodies of 

 certain vertebrates, being more destructible than bones and teeth, are 

 not often preserved in a fossil condition except as imprints or casts. 

 This remark, however, does not apply to the scales of teleost fishes, 

 which, although epidermal in their character, are nearly or quite as 

 indestructible as are the bones and teeth of those animals. 



Although the members of the orders Cetacea and Sirenia, and of the 

 family Phocid** of the order Carnivora, are very numerous, they con- 

 stitute only a small proportion of the whole class Mammalia, and it is 

 these families alone every member of which is fully adapted to an aquatic 



"The small family to which the walruses belong should also he included here. 



