INTRODUCTION 



19 



Since muscles and bones make up the main bulk of the animal, 

 a restoration to living proportions is possible. The shape of the 



teeth tell the character of its 



diet, which in turn reveals 

 the size of the digestive or- 

 gans. The superficial cover- 

 ing, however, is often a 

 matter of conjecture. In 

 fish the covering of scales is 

 usually preserved ; in amphi- 

 bia mucous gland impressions 

 often show upon the surface 

 of the bones, thus indicating 

 a soft, slimy skin for the 

 animal ; in reptiles the pres- 

 ervation of scales or bony 

 denticles or their impressions 

 indicate the character of the 

 external covering (Fig. 5). 

 When, however, as often 

 happens, nothing of the sort 

 is preserved, the restoration must be based upon comparison 

 with fossil and living forms ; especially suggestive are the young 

 of nearly related living species. Studies in evolution have 

 shown that each individual in its development from the egg 

 to maturity passes through stages which are similar in general 

 to its successive adult ancestors from earliest geologic ages to 

 the present. Hence a stage in the youth of a living animal 

 would be suggestive of related extinct forms. This principle, 

 however, must be applied with the greatest caution. The adult 

 elephant and rhinoceros are almost hairless, while the young 

 have much hair ; that this may be reminiscent of the ancestral 

 forms is suggested by the Pleistocene hairy mammoths preserved 

 in the Siberian ice fields. The coloration of the young likewise 

 is difficult of explanation unless it be taken as representing the 



Fig. 5. — Natural mold of the surface 

 markings of the dinosaur, Stephano- 

 saurus marginatus Lambe, from the 

 Cretaceous of Alberta. These mark- 

 ings are from the side of the body, 

 where the yielding mud receiving the 

 impression of the skin of the reptile 

 upon its burial, has preserved them 

 intact though the skin itself has long 

 since disappeared. (From Lambe.) 



