president's address. 45 



ably important part in the evolution of the different forms of animal 

 life which have existed upon the earth. Second, in those cases of 

 land-locking of marine animals by a rise of the sea bottom in the 

 manner already suggested, only a part of the fauna then existing 

 there would probably have become inclosed, because many of them 

 would no doubt have escaped into the outer sea before they were 

 fully surrounded by land. Third, only a part of those which were 

 finally land-locked were able to survive the change from salt to 

 fresh water. Fourth, a large proportion of marine gill-bearing 

 animals appear to be, and always to have been, wholly incapable of 

 living in fresh water. 



Among those marine animals which seem to have been wholly or 

 mainly incapable of surviving a change of habitat to fresh waters, 

 and which, we may assume, did not escape land-locking, together 

 with the other forms, in the numerous cases of the kind which have 

 occurred in past geological periods, are several of the entire compre- 

 hensive groups into which the animal kingdom is divided. For exam- 

 ple, we learn from the study of existing faunas that, with the excep- 

 tion of a few inconspicuous forms of the Coelenterata, which are so 

 abundantly represented in marine waters, this important sub-king- 

 dom is not represented in any fresh waters ; also, in fresh waters the 

 entire classes, Echinodermata, Tunicata, Brachiopoda, Pteropoda, 

 and Cephalopoda, are without any known representation. Besides 

 these large deficiencies in fresh-water faunas, as compared with 

 those of the sea, there are numerous minor, but no less important, 

 deficiencies, occasioned by the entire absence of a considerable 

 number of orders and families, as well as parts of others. 



It would doubtless be unsafe to say that any of those animals 

 could not possibly have survived a change to a fresh-water habitat; 

 but there is much reason to suppose that they possess some inherent 

 quality, which has prevented the survival of their ancient represen- 

 tatives, whenever their habitat may have been changed from a 

 marine to a fresh-water condition. Indeed, if such changes of 



aqueous condition had been effected suddenly, it is probable that 

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