456 LECTURE XVI. 



of fishes ! Mammals in tlie Jurassic formation^ mammals in the 

 Purbecklimestonej which some consider asbelonging to the lowest 

 chalk. AdieUj then, Empire of reptiles ! Man in the upper tertiary 

 strata ; man in the diluvium ; good-bye Empire of mammals ! 



The proof of one well-attested transitional form includes the 

 possibility of all other transitional forms, but not their necessity 

 or actual existence. 



I must now draw your attention to another point rendered evi- 

 dent by these examples. The transitional forms between the two 

 bear species with fijced characters — the cave bear and the brown 

 bear, are as rare as the two species themselves are abundant. One 

 portion of them seems also to have been intermediate in time, 

 since the colossal brown bears from the AljDine caverns of Swit- 

 zerland are more recent than the cave-bear, which is, according to 

 Wagner, also the case as regards the Ursus priscus, whose head 

 is found along with the lower jaw, and thus probably deposited 

 by standing water, whilst the cave-bear skulls are never found 

 so connected. But apart from this circumstance, we must 

 notice the rarity of such transitional forms. There is no doubt, 

 if the change of surrounding media occurs within a compara- 

 tively short time, the transformation of the type must keep 

 pace with it. We have alluded to this when speaking of the 

 domestic animals introduced into South America. The modi- 

 fication of type experienced by cats in Paraguay, swine in 

 Chile and the Brazils, and the sheep in the same regions, in 

 consequence of a sudden transplantation, were effected within a 

 few generations. The transformed type was soon adapted to 

 the climate, and has now assumed a stationary form. Unfor- 

 tunately our knowledge refers only to the exterior, and not- 

 withstanding the great interest attached to this question, no 

 naturalist has yet compared the skulls of the European domes- 

 tic animals, acclimatized in South America, with those of the 

 parent stocks bred in Europe. Admitting that striking dif- 

 ferences do exist, that the skull of the swine, for instance, 

 has become shorter and higher, the snout thicker, the tusks 

 more curved, so that the South American domestic pig repre- 

 sents, even in the skeleton, a new and easily distinguished 

 species. Admitting all this, where do we find the transi- 

 tional form which led to this result ? — Nowhere ! The millions 



