THE RISE AND PROGRESS OF PALEONTOLOGY. 173 



fishes termed ganoids is at the present time so distinct from that of 

 the dipnoi or mud -fishes that they have been reckoned as distinct 

 orders, the Devonian strata present us with forms of which it is im- 

 possible to say with certainty whether they are dipnoi or whether they 

 are ganoids. 



Agassiz's long and elaborate researches upon fossil fishes, published 

 between 1833 and 1842, led him to suggest the existence of another 

 kind of relation between ancient and modern forms of life. He ob- 

 served that the oldest fishes presented many characters which recall 

 the embryonic conditions of existing fishes ; and that, not only among 

 fishes, but in several groups of the invertebrata which have a long 

 paleontological history, the latest forms are more modified, more spe- 

 cialized, than the earlier. The fact that the dentition of the older ter- 

 tiary ungulate and carnivorous mammals is always complete, noticed 

 by Professor Owen, illustrated the same generalization. 



Another no less suggestive observation was made by Mr. Darwin, 

 whose personal investigations during the voyage of the Beagle led 

 him to remark upon the singular fact that the fauna which immedi- 

 ately precedes that at present existing in any geographical jn-ovince 

 of distribution presents the same peculiarities as its successor. Thus, 

 in South America and in Australia, the later tertiary or quaternary 

 fossils show that the fauna which immediately preceded that of the 

 present day was, in the one case, as much characterized by edentates 

 and in the other by marsupials as it is now, although the species of 

 the older are largely different from those of the newer fauna. 



However clearly these indications might point in one direction, the 

 question of the exact relation of the successive forms of animal and 

 vegetable life could be satisfactorily settled only in one way namely, 

 by comparing, stage by stage, the series of forms presented by one 

 and the same type throughout a long space of time. Within the last 

 few years this has been done fully in the case of the horse, less com- 

 pletely in the case of the other principal types of the ungulata and of 

 the carnivora, and all these investigations tend to one general result 

 namely, that in any given series the successive members of that series 

 present a gradually increasing specialization of structure. That is to 

 say, if any such mammal at present existing has specially modified and 

 reduced limbs or dentition and complicated brain, its predecessors in 

 time show less and less modification and reduction in limbs and teeth 

 and a less highly developed brain. The labors of Gaudry, Mai-sh, and 

 Cope furnish abundant illustrations of this law from the marvelous 

 fossil wealth of Pikermi, and the vast uninterrupted series of tertiary 

 rocks in the Territories of North America. 



I will now sum up the results of this sketch of the rise and prog- 

 ress of paleontology. The whole fabric of paleontology is based 

 upon two propositions : the first is, that fossils are the remains of ani- 

 mals and plants ; and the second is, that the stratified rocks in which 



