408 Causes and Course of Organic Evolution 



of the surf may beat its inhabitants in pieces, the retreat of 

 the tide exposes them, not merely to innumerable enemies in 

 the shape of predatoiy birds and beasts, but also to a change 

 in the atmospheric medium by which they are surrounded." 



Later (p. 11) he reaches the conclusion: "The ultimate 

 derivation of the whole of the land and fresh-water molluscan 

 fauna must, as has already been remarked, be looked for in 

 the sea." These arguments have strong weight, at least for 

 large groups of the Mollusca. But we must place against 

 such the fact that even the highly developed scorpions and 

 insects were already evolved during the carboniferous period. 

 Further the enormous development in species and genera 

 wliich the ''worms" and the land arthropods have undergone 

 is proof that a fresh-water or land environment is prolific of 

 organic change, even though the strain on life — the struggle 

 for existence — there is greater than in the sea. Accordingly 

 proenvironal response and selective survival act more sharply 

 to evolve new types. 



The extensive and often rapid transfer of marine forms Vjy 

 sea currents, or by attachment to other sea animals, are unques- 

 tionably important factors in spreading these abroad. Where 

 groups therefore like the Spongida, the Echinodennata, or 

 the ]\Iollusca become preponderatingly marine, they branch 

 out into a great variety of types. But such are not the groups 

 which seem to have steadily evolved into higher and specially 

 alert organisms, that by quick nerv^ous movement could equally 

 escape from enemies or untoward physical conditions, and 

 also secure food supplies most quickly and extensively. 



No one we believe would now doubt that the Sirenia and 

 Cetacea are greatly modified marine forms that have originated 

 from a terrestrial ancestry amongst the mammals. And yet 

 they are as greatly modified morphologically from what must 

 have been the originating land type as many of the marine 

 crustaceans and molluscs are and have been from fresh-water 

 types that we would regard as closely resembling their ancestral 

 originators. But the close fundamental agreement between 

 larval forms of many ccelente rates, ecliinoderms, worms, and 



