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 fœtal 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 prenatal 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 ecenogenesis 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 conflicts between the two records. He will on the whole have in embryology a fair mirror of the past wherein, even though the image be somewhat distorted and the more remote reflections dimmed by time, he can view the striking features of the long procession of the ages.
