RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 255 



life. A considerable number of other mammals, such as various car- 

 nivores, rodents, etc., are amphibious in their habits, but far the greater 

 part of this important class are dwellers upon the land. Therefore, while 

 remains of the extinct representatives of the aquatic animals just 

 mentioned would naturally have become intombed in the sediments of 

 the waters in which they lived, and have there become fossilized, 

 remains of representatives of the strictly land mammals could have 

 reached a similar intombment only in the indirect manner that has 

 already been explained. That is, the intombment of the. remains of 

 the aquatic mammals was almost a matter of course, while that of the 

 remains of all others was the result of the exceptional and accidental 

 falling or conveyance of their bodies into the water after death, or of 

 their miring and dying in the slime of ponds and marshes. 



The greater part of the remarks which have just been made con- 

 cerning mammals is applicable to birds, and perhaps in some respects 

 with even greater force, for it is doubtful if so large a proportion of 

 formerly existing birds as of mammals have become represented by 

 fossil remains. Only a small proportion of now existing birds habitually 

 live upon the water, and these, like all others, nest upon the land. The 

 remains of at least a portion of those which habitually resort to the 

 water are of course likely to become quickly intombed in its sediments, 

 while remains of all strictly land birds must reach such intombment, 

 if at all, by indirect or accidental means. Therefore, fossil remains 

 of aquatic birds are more likely to be discovered in sedimentary rocks 

 than are those of any others, although it is quite probable that the 

 terrestrial kinds as greatly preponderated over aquatic kinds in former 

 geological times as they now do. 



While all reptiles are air-breathers many of them habitually live in 

 the water and in adjoining swamps and marshes. Many of this class, 

 however, are not only confined to the land, but some of them abound in 

 arid districts. The preservation of reptilian remains is, of course, sub- 

 ject to conditions similar to those under which mammalian and avian 

 remains are preserved, and it is therefore evident that while remains of 

 aquatic and palustral reptiles may readily find sedimentary or slimy 

 intombment those of strictly land reptiles are less likely to become thus 

 preserved. It is doubtless in part for this reason that fossil remains of 

 representatives of now living upland reptiles are so rare as compared 

 with those of representatives of other living forms. It is true that alarge 

 proportion of the great extinct subclass of Dinosauria were vegetable 

 feeders, as is shown by their skeletal structure and especially by the 

 character of their teeth, but most of those whose remains have been 

 discovered were probably of lowland or palustral rather than of upland 

 habitat. Their remains were therefore more likely to have undergone 

 intombment than were thoseof the upland reptiles which may have been 

 contemporary with them. That is, besides the usual methods in the 

 case of land animals, their remains were liable to intoinbmeut by miring 



