458 VERTEBRATA IN THE [Ch. XXVIL 



above table (p. 456), are due to British skill and energy, Great Britain 

 being still the only country in which mammalia have been found in 

 Oolitic rocks ; the only region where any reptiles have been detected in 

 strata as old as the Devonian ; the only one wherein the bones of birds 

 have been traced back as far as the London Clay. And, if geology had 

 been cultivated with less zeal in our island, we should know nothing as 

 yet of two extensive assemblages of tertiary mammalia of higher antiquity 

 than the fauna of the Paris gypsum (already cited as having once laid 

 claim to be the earliest that ever flourished on the earth) — namely, first, 

 that of the Headon series (see above, p. 212) ; and, secondly, one long 

 prior to it in date, and antecedent to the London Clay.* This last has 

 already afforded us indications of Quadrumana, Cheiroptera, Pachyder- 

 mata, and Marsupialia (see p. 217). How then can we doubt, if every 

 area on the globe were to be studied with the same diligence, — if all 

 Europe, Asia, Africa, America, and Australia were equally well known, 

 that every date assigned by us in the above Table for the earliest recorded 

 appearance of fish, reptiles, birds, and mammals would have to be altered ? 

 Nay, if one other area, such as part of Spain, of the size of England and 

 Scotland, were subjected to the same scrutiny (and we are still very im- 

 perfectly acquainted even with Great Britain), each class of Vertebrata 

 would probably recede one or more steps farther back into the abyss of 

 time : fish might penetrate into the Lower Silurian, — reptiles into the 

 Lower Devonian, — mammalia into the Lower Trias, — birds into the 

 Chalk or Oolite, — and, if we turn to the Invertebrata, Trilobites and 

 Cephalopods might descend into the Lower Cambrian, — and some stray 

 zoophyte, like the Oldhamia, into rocks now styled " azoic." 



Yet, after these and many more analogous revisions of the Table, it 

 might still be just as easy as now to found a theory of progressive devel- 

 opment on the new set of positive and negative facts thus established ; 

 for the order of chronological succession in the different classes of fossil 

 animals would probably continue the same as now ; — in other words, our 

 success in tracing back the remains of each class to remote eras would be 

 greatest in fishes, next in reptiles, next in mammalia, and least in birds. 

 That we should meet with ichthyolites more universally at each era, and 

 at greater depths in the series, than any other class of fossil vertebrata, 

 would follow partly from our having as paleontologists to do chiefly with 

 strata of marine origin, and partly, because bones of fish, however partial 

 and capricious their distribution on the bed of the sea, are nevertheless 

 more easily met with than those of reptiles or mammalia. In like man- 

 ner, the extreme rarity of birds in recent and Pliocene strata, even in those 

 of freshwater origin, might lead us to anticipate that their remains would 

 be obtained with the greatest difficulty in the older rocks, as the Table 

 proves to be the case, — even in tertiary strata, wherein we can more 

 readily find deposits formed in lakes and estuaries. 



* A bird's bone is recorded as having been lately found in the Woolwich 

 beds (beneath the London clay), by Mr. Prestwich ; GeoL Quart. Journ. vol. x. 

 p. 157. 



