Introduction 7 



on, the above parts are at times encountered in enormous 

 quantity and in a beautiful state of preservation. Sixth, 

 the group has furnished so large a share of human food 

 that it has received much careful and detailed attention. 



Such combined conditions are possessed by few if any 

 other divisions of animals. But the writer might add that 

 he hopes soon to publish data that will serve equally well 

 to elucidate the past and present history of the Crustacea. 

 Still later it may be possible to treat the Mollusca in 

 a similar manner, thougih the effort promises to be a 

 more extended one. But should some student of this last 

 great and diverse assemblage be inclined to undertake the 

 work alike from the embryological, the morphological, 

 the ecological, the taxonomic and palaeontological side, the 

 results will unquestionably be of highest value. 



In undertaking such a study as the present, the writer 

 is fully conscious of his own limitations and shortcomings, 

 but would plead in fullest extenuation that having reached 

 certain published conclusions it is incumbent on him to 

 verify or to disprove the statements already made. The 

 field is a wide one, the evidences must be drawn from many 

 and diverse sources of human knowledge, the conclusions 

 reached must be fortified by abundant and exact obser- 

 vations, such conclusions also must fit in with like obser- 

 vations made for other groups of animals than the fishes, 

 as well as for plants. Finally the facts obtained from 

 study of fossil forms must be correlated with, and must 

 lead up to, a correct interpretation of the data already 

 gathered as to living fishes. 



So in the immediately succeeding chapters consideration 

 is given to such general questions as the changes of the 

 earth before and during the period of the evolution of 

 fishes; the possible relation of recent to fossil fishes; the 

 environal conditions by which evolving fishes were sur- 

 rounded; the invertebrate predecessors and ancestors of 

 fishes; also the succession of forms observed during 

 successive epochs. Only thus is it possible to obtain a fairly 

 concrete and accurate picture of the group as a whole. 

 Such treatment also forms a fitting introduction to still 

 later chapters, which deal with the great subdivisions of 



