50 THE PALEONTOLOGIC RE COED 



sion develop into the powerful swimming flukes of the adult. They 

 may be compared with lateral flanges on the tail of the sea otter 

 Enhydris, but in the latter the flaps are elongate, while in the Cetacea 

 they are short and situated toward the end of the tail. Nevertheless, 

 the homology of the two types of flange structures appears true, the 

 posterior position and concentration in the whale being a mechanical 

 adaptation which has become accelerated in its appearance so as to be 

 embryonic. The presence of hair on the body of the foetal whale and 

 of distinct calcareous tooth germs in both upper and lower jaws of the 

 unborn young of whalebone whales are both reminiscent. 



The horses, our knowledge of which is so complete owing to the 

 pioneer work of Marsh and later of Osborn, show some interesting 

 points of comparison between foetus and ancestor. The skulls of pre- 

 natal modern horses resemble those of Mesohippus or even of Eohippus 

 in the proportions of face and cranium, the short-crowned grinding 

 teeth, lesser angle between basi-cranial and basi-facial axes and the fact 

 that the orbit is incompletely ringed with bone. The feet of the unborn 

 foal are also somewhat reminiscent of old-time conditions. 



One of the most difficult points to be reconciled in the acceptance 

 of the Cope-Osborn theory of the origin of molar cusps was the apparent 

 non-agreement of cusp ontogeny with the interpreted phylogeny which 

 this theory upheld. The difficulty has been met in two ways: by the 

 supposition that coenogenesis has entered into the embryogeny, or that 

 the paleontological record as shown by the trituberculists is open to a 

 different interpretation. The present great exponent of the idea claims 

 that the matter is still sub judice and thus the problem stands. 



In conclusion, the paleontological student of the higher vertebrates 

 can hope to find in embryology a host of valuable suggestions, much 

 verification of his work and sundry apparent inconsistencies which must 

 in some way be reconciled. He should ever bear in mind the influence 

 of nature and nurture, the latter often giving rise to perplexing con- 

 flicts between the two records. He will on the whole have in embry- 

 ology a fair mirror of the past wherein, even though the image be some- 

 what distorted and the more remote reflections dimmed by time, he can 

 view the striking features of the long procession of the ages. 



