MATTHEW, CLIMATE AND EVOLUTION 183 



being replaced by new invasions from the land instead of evolving further 

 in their new habitat. The ichthyosaurs, plesiosaurs, mosasaurs, sea- 

 crocodiles, sea-turtles, are examples of this sort among reptiles ; the ceta- 

 ceans and seals among mammals. These invasions from a higher to a 

 lower plane of active life have been very frequent, so that their recogni- 

 tion is necessary in tracing evolutionary series. The converse movement 

 from a lower to a higher plane, as from aquatic to amphibious, from 

 amphibious to terrestrial, from terrestrial to arboreal or aerial, have been 

 slow, difficult and for the most part have occurred but once or twice in 

 the geological history of vertebrate life. The higher field once occupied, 

 the lower adaptation was handicapped in its attempts to rise. 



Imperfection of the Geological Kecokd 



Everyone is familiar with Darwin's classic illustration of the imper- 

 fection of tlie geological record ;^*^ but I doubt whether the majority of 

 paleontologists realize how very imperfect our record is, even to-day. We 

 know more about fossil mammals in' proportion to their modern numbers 

 than about any other of the larger groups of land animals ; yet the num- 

 ber of species of which we have any adequate knowledge is but a minute 

 fraction of the number which must have lived since the class first came 

 into existence. AVere it not so, the fossil species would vastly outnumber 

 the living forms; as it is, they form a small minority. Moreover, the 

 greater number of recorded fossil species are hardly more than nomina 

 nuda, each known from a single fragToentary jaw, a tooth, a scale, a 

 broken bone, indicating indeed that an animal otherwise unknown lived 

 at a certain time in a certain locality but giving very little information 

 as to its entire structure, its habits, its geographical and geological range. 

 The relationships of these imperfectly known species, provisionally stated 

 by the describers and adopted without the query by sul)sequent writers, 

 are one of the most fertile sources of error in paleontological theories. 



Mammals undoubtedly existed during the entire Mesozoic, an era about 

 three times as long as the Cenozoic. Two thirds of their evolution must 

 have taken place during that time; and by the end of it, the principal 

 modern orders were already defined. But we have not a skeleton, or even 

 a skull of a single Mesozoic mammal.^^ Two jaws and a few teeth from 

 the Triassic, a number of more or less fragmentary jaws from tbe upper 

 Jurassic and various teeth and fragments of jaws from tlie uppermost 

 Cretaceous represent the sum total of our real knowledge of the first two 



» In the Origin of Species, at the end of Chapter X. 

 11 Setting; aside Tritylodon as of doubtful affinities. 



