43 



of them. A discussion of formerly existing vegetable forms would 

 also be of great interest, but I must omit all except incidental 

 reference to that subject also. 



In all investigations into the history of ancient life upon the 

 earth, regard must be had to the functions that animals perform, 

 and to the conditions under which those functions are executed. 

 The requisite conditions for the performance of the physiological 

 functions in the simplest animal forms, the Protozoa, for example, 

 render it practically certain that the primary origin of animal life 

 occurred in water ; and it doubtless occurred in the sea. The first 

 animal life having necessarily been of aqueous origin, we must 

 assume that the first air-breathing animals were developed from 

 those of aqueous respiration. 



Fresh-water mollusks and fishes, especially the former, have, I 

 believe, primarily become such by a change from their originally 

 marine habitat, mainly by compulsion; that is, their progenitors 

 lived in the sea and became land-locked by the unequal elevation 

 of the sea bottom upon, or over which, they lived while the conti 

 nental areas were in process of eleva*tion. The waters of the dis 

 tricts thus inclosed and elevated above the level of the surrounding 

 sea became first brackish, and then fresh, in consequence of the 

 influx of fresh water from the drainage of the surrounding land, 

 and a consequent outflow into the open sea. Those of the sea-born 

 animals which became thus inclosed, and which were capable of 

 conforming to the new conditions, did so, and peopled the river 

 systems which were produced in connection with, and which suc 

 ceeded, these fresh-water lakes. Those which could not thus con 

 form to the new conditions became extinct ; and as these appear to 

 have constituted the larger part of every fauna which became land 

 locked in the manner referred to, we may reasonably conclude that 

 the lines of descent of many of the groups of marine animals have 

 been broken by this means. 



River systems have resulted upon the disappearance, by final 

 drainage, of the fresh-water lakes just mentioned, the inlets having 



