]82 ANNALS Nnw YORK ACAniniY OF SCIENCES 



liiu' ol' diiiuf^auriaii r\(jliitiuii inliabitcd the ii])laiids, where the sediments, 

 if sucli were deposited, have long since been removed by erosion,, and the 

 raiiiia is consequently unknown to us, except by inference. It is quite 

 impossible to trace the evolution of the dinosaurian phyla through the 

 same nearly direct series of known forms as can be done in tlie ])hy]a of 

 Tertiary mammals. But I may observe that if our knowledge of the Ter- 

 tiary sediments were limited to the coastal 'swamp deposits, — if in this 

 country, for instance, we knew only the Tertiary of the Atlantic and 

 Gulf coasts, — we would be equally at a loss for any direct ancestral series 

 illustrating the evolution of the Mammalia. 



The same explanation, namely, that the geological recorcl in the Meso- 

 zoic is defective where its evidence would be most direct as to the evolu- 

 tion of land vertebrates, applies both to birds and to mammals, but espe- 

 cially to the former. The exceeding scantiness of fossil birds and mam- 

 mals during the Mesozoic and their apparently sudden appearance in the 

 record, already well deployed, is often explained by supposing them to 

 have evolved mainly in some continent not yet investigated. It appears 

 to me that a simpler and more pi'obable explanation lies in the fact that 

 the formations of the interior of the Mesozoic continents have in general 

 not been preserved and that this facies of the Mesozoic faunae is conse- 

 quently unknown to us. 



It may be objected that remains of dry-land animals would be brought 

 down by rivers and deposited in their deltas and thus preserved to our 

 day. This may, of course, occur in exceptional cases. How rare is the 

 exception, we may judge from the exceeding rarity of remains of land 

 animals in true marine deposits, where the chances for their preservation 

 should be almost equally great. 



Ill marked contrast with the evolutionary record among dinosaurs, 

 stands the record of development of the noir-marine crocodiles and che- 

 lonians, whose normal habitat was the swamp regions and whose more 

 direct evolution is in consequence recorded since the Mesozoic. Remain- 

 ing in a constant environment, they evolved but little, though their abun- 

 dance and geographical distribution varied. 



Throughout all the evolutionary history of the vertebrates, we see 

 numerous examples of races which, having become adapted to a higher 

 ])lane of life, have re-invaded a lower plane. In each instance, the higher 

 organization and greater activity acquired in the higher plane have caused 

 them to become dominant, increase rapidly in size and spread widely in 

 the absence of efficient competition. Thus we find various groups of ma- 

 rine reptiles appearing with apparent suddenness in the Mesozoic, becom- 

 ing very abundant and of gigantic size, spreading very widely and then 



