Introduction 19 



and others, while repeated descriptions have been made of 

 supposed natural hybrids between distinct species, as for ex- 

 ample with the snappers {Liitianus) of Cuba. So far as 

 at present known however, these are rare and exceptional 

 conditions. 



But it should be borne in mind that we are still largely 

 ignorant as to the possible effect of changed environment 

 on the eggs and young of nemerteans and fishes. Graham 

 Kerr has pointed out (5:8) that this embryonic stage may 

 also be the most susceptible in the history of individuals 

 and species. An extensive field for experimental study is 

 here open. 



Having briefly reviewed the above important biologic 

 conditions it can now be stated that the four fundamental 

 theses of the present volume are: (a) that the Nemertinea 

 are the progenitors of all the chordate groups, including 

 the Hemichordata, the Urochordata, the Cephalochordata 

 and Euchordata, (b) that the freshwater and land Meta- 

 nemerteans of the present day represent direct descendants 

 of related forms that evolved from simpler though envi- 

 ronally similar freshwater types, but which also gave off 

 outliers that passed into marine surroundings and there 

 evolved numerous new genera and species; (c) that from 

 the higher freshwater Metanemerteans the Euchordata 

 developed, and long remained wholly or almost wholly 

 in like freshwater environment as their metanemertean 

 ancestors, (d) that in process of this development the 

 Euchordata early separated — probably in late Cambrian 

 or early Ordovician times — into two divergent groups, the 

 Cyclostomata and the Gnathostomata, and that representa- 

 tives of both still exist. Though outside the field of the 

 present work, but already dealt with by the author (/: 

 454-474) it might be added that he regards freshwater 

 cyclostomes allied to Petromyzon or some nearly related 

 and diverging series, as the progenitors of the Amphibia 

 and still later of the Mammalia. A few words can now 

 be added regarding the four above positions, preliminary 

 to their detailed consideration in subsequent chapters. 



(a) Under the first heading it can be said that the 

 Metanemertinea is the only group of invertebrates which 



