196 From Matter to Man. 



we anticipate that no great complexity of mechanism 

 could be attained, even by natural selection, on those 

 suicidal lines alone. 



Mollusca. — In the mollusca we have two distinct 

 types of shell-fish — the bivalve and the univalve — but 

 in both the mechanism is crude. For purposes of 

 locomotion, the bivalve's machinery, as in the oyster, 

 is practically useless, motion consisting almost solely 

 in opening and closing the shell. Its life mechanism, 

 on the other hand, is complex, comprising a stomach, 

 gills both for receiving food and respiring, and quite 

 a number of coloured eye spots set round the borders 

 of its mantle, each organ necessarily fitted with proper 

 muscular and nervous machinery. Razor-shells can 

 dig, mussels anchor themselves to the rocks with their 

 slender muscular organs called feet, while cockles 

 jump by means of the same. We can conceive, 

 however, of no very complex motor developing from 

 the plan of imprisoning an animal in a bivalve shell. 

 Lastly, although seemingly capitally constructed for 

 defence, bivalves are not invulnerable, for they are at 

 the mercy of such an enemy as the whelk, who, with 

 his rasp, bores a hole through their armour and 

 lunches at his leisure. Univalve shell-fish display 

 more locomotion ; still, a snail is not a synonym for 

 activity. The necessity of carrying their heritable 

 property on their back is rather a handicap in the 

 race of life and the survival of the fittest. Progress, 

 moreover, is accomplished by a lengthening and 

 .shortening of one's self ; a mode of motion necessarily 



