THE PALEONTOLOGIC RECORD 153 
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 Hohippus 
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 ccenogenesis hag 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 swb 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. 
VOL, LXxvilI.—l1. 
