Evolution of Animals 407 



In this connection however it is well to remember that owing 

 to active volcanic agency, and the activity of hot springs and 

 other thermal influences from the archsean to the silurian 

 epochs, many land-locked water areas, and probably even 

 streams, must have been charged with free acids, salts, and 

 other constituents much as are some of the siliceous, carbon- 

 ated, iron, and other areas of streams at the present day. The 

 variety of these, their comparative frequency, and the adapta- 

 tion of many organisms to such waters, would either give an 

 elastic adaptability to the organisms, as is still shown by many 

 species or genera of the present day, or it would accustom 

 some organisms by slow but sure stages to a marine existence. 



If we attempt to estimate the number and variety of envir- 

 onal agents that fresh-water, marine, and land animals are 

 relatively exposed to, it may safely be said, we believe, that 

 fresh-water species are most exposed throughout the year — 

 though not perhaps during each successive day or week — to 

 varying environal changes. The inhabitants of shore lines 

 and estuaries have frequently sharp daily changes, but the 

 more extended periodic changes are averagely less marked 

 probably than for the occupants of lakes and rivers. 



In this connection Cooke says in his discussion of molluscan 

 distribution {133: 1): "It is the generally accepted opinion 

 among men of science that all life originated in the sea. Not 

 that all parts of the sea are equally favorable to the develop- 

 ment of forms of life. The ocean surface with its entire ab- 

 sence of shelter or resting place, and the deep sea, whose abysses 

 are always dark and cold and changeless, offer little encourage- 

 ment to plant or animal life, as an original starting point." 

 And again: "It was no doubt the littoral region and the shallow 

 waters immediately below it, a region of changeable currents, 

 of light and shade, of variation, within definite limits, of tem- 

 perature and tide effects, which became the scene of the original 

 development of plant life, in other words, of the food su])i)ly 

 wliich rendered possible its colonization by higher animals. 

 But the littoral region, besides the advantages of tenancy which 

 it offers to animal life, has also its drawbacks. The violence 



