NATURAL SCIENCES OP PHILADELPHIA. 277 



onveutlasubdiviserenordresd'apresdescaracteresfixesetsensibles.''* Never- 

 theless, it is well known that Agassiz, abandoning the Cuvierian method of com- 

 paring animals by their organs, and adopting Bichat's scheme of comparing the 

 tissues of oryans instead, was enabled to reconstruct the fishes of tbe fossil 

 world by noting carefully the characteristics of their tegumentary membrane. 

 If it be true, indeed, for the animal world at large, as maintained by Knox, 

 that specific characters are in the main external ; and that the anatomy of the 

 interior leads to higher considerations than the mere determination of species ; 

 and if it be true, that, on this account, the law of correlation so often fails in 

 its application to species, still more should it fail when used as a means of 

 diagnosticating human crania from each other. For a serial unity of form is 

 here more manifest than in the animal "world proper, and this unity has 

 become still more apparent under the combined influence of civilization and 

 hybridity. In long periods of time civilization appears to be capable of 

 modifying human cranial forms to a slight though appreciable extent. Hy- 

 bridity, by introducing intermediate or transitionary forms, gives to osteological 

 characters, originally differential, an uncertain or fluctuating value. Natural- 

 ists are not agreed whether the carnivora of the fossil world were identical 

 with the lions, tigers, panthers, leopards, &c, of the present time, or were 

 specifically distinct from these. They are not yet decided whether all the 

 species of the present fauna of this continent are distinct from those found 

 fossil in the post-pliocene deposits of South Carolina or not. They find that 

 the teeth and bones of the living rabbit, raccoon, opossum, deer, elk, hog, 

 dog, sheep, ox and horse, cannot be distinguished anatomically from similar 

 remains found in these deposits, and they are consequently at a loss whether 

 to regard the former as the direct descendants of the latter, or entirely distinct 

 from them ; and this, too, notwithstanding that the fossil specimens are found 

 associated with the remains of animals positively known to be extinct, such 

 as mastodon, megatherium, hipparion, &c.f They are not agreed whether the 

 fossil horse resembled the quagga, the zebra, the dzigguetai, the domestic 

 horse, or an animal wholly and specifically distinct from all these. Agassiz 

 "entertains doubts respecting the unity of origin of the domesticated horse. "j 

 According to Knox, the fossil horse belongs to no species of this animal now 

 living. Prof. Owen finding that one of the teeth of a certain fossil horse is 

 6omeWhat more curved than the corresponding tooth of the recent horse, 

 declares the former to be a distinct species, and names it Equus curvidens. 

 Prof. Leidy is persuaded that many remains of an extinct species of horse, 

 from the post-pliocene of this country, are undistinguishable from the recent 

 one. The specimens of teeth of this animal, which he has had the oppor- 

 tunity of exhibiting, present so much difference in condition of preservation 

 or change in structure ; so much variation in size, from that of the more 

 ordinary horse to the largest English dray horse ; and so much variableness 

 in constitution, from that of the recent horse to the most complex condition 

 belonging to any extinct, species described, that it would be about as easy, he 

 thin ! s, to indicate a h If dozen species as it would two. || So it is with the varied 

 cranial forms displayed in the great natural family man. Of human crania, it 

 is just as easy, indeed, I think it is easier to make twenty-seven races, types, 

 permanent varieties, or species call them what you will as it is to make 

 any less number so very mobile, so very elastic is the fundamental plan or 

 structural type of the human skull. The uncertainty which surrounds the 

 definition of the species of the genus Equus, exists also in connection with the 



*Regne Animale, 1, ii p. 28. 



fSec Proceedings Acad Nat. Sci., July 1859, p. 184. 



{See hi* letter addressed to Prof. Holmes, in Proc. Acad. Nat. Sci., July 1859, p. 18f. 

 ^Introduction to Inquiries into the Philosophy of Zoology, in London Lancet, for 

 October, 1855, p. 275. 

 ||Proc. Acad. Nat. Sci., July, 1859, p. 182, 



1859.J 



