MESOZOIC AND CENOZOIC FISHES 229 



always retained their primitive remote situation, and the fin-rays never 

 became spines. During the Cretaceous period the majority of the bony 

 fishes began to exhibit modifications in all these characters, and the 

 changes occurred so rapidly that, by the dawn of the Eocene period, the 

 diversity observable in the dominant fish fauna was much greater than 

 it had ever been before. At this remote epoch, indeed, nearly all the 

 great groups of bony fishes, as represented in the existing world, were 

 already differentiated, and their subsequent modifications have been of 

 quite a minor character.'' 



The general course of evolution observable among fishes being as above 

 briefly summarized, it has been the aim of specialists during the past 

 decade to work out the details of classification with ever increasing accu- 

 racy, as far as the existing state of our knowledge will permit. A further 

 object of study has been to gather more precise information regarding the 

 anatomical structure of the different forms whose remains have been 

 preserved, whether in the entire, or crushed and fragmentary condition ; 

 and towards this end a vast deal of material has been accumulated. Still 

 another objective point, and one more difficult of attainment, has been 

 to trace the stages of differentiation passed through by the dominant 

 Tertiary fishes, which in turn gave rise to the modern fauna. 



In connection with the latter problem, an interesting allied topic has 

 presented itself for investigation, namely, that of the antiquity of the 

 deep-sea fish fauna. It is now well recognized that among marine organ- 

 isms many forms which have been worsted in the unceasing struggle for 

 existence — waged always most keenly along the shorelines of conti- 

 nents — manage to survive through having emigrated , elsewhere, amid less 

 trying conditions. Divers groups of animals, for instance, have been 

 forced to seek a refuge in fresh waters, and have there continued to per- 

 sist, some dwindling, others flourishing, according to their ability to 

 react in response to changed environment. Other groups, and among 

 them many fishes, have been driven into tenanting the abysmal depths 

 of the ocean. 



Paleontology shows that the last mentioned refuge was not inhabited 

 to any great extent by fishes prior to the latter part of the Cretaceous. 

 But, beginning during this period and steadily proceeding until the 

 present day, a gradual migration of certain groups of fishes into great 

 depths of the ocean has been in progress, coincident with remarkably 

 striking changes in the anatomical structure of the emigrant outcasts. 

 As a result of recent researches, more especially of the late Cretaceous 

 and Eocene deep-sea fish faunas, we are enabled to note the gradually 

 changing constitution of these abyssal assemblages from the close of the 



