24 THE STORY OF ANIMAL LIFE 



move. For the flat valves of the shell are placed 

 to the right and left of the animal's body. So 

 that when it stirs, or floats quietly in the cur- 

 rent of the tide, the shells present their sharp 

 edges to the resistance of the water, thus enabling 

 the creature to move like a ship through the sea, 

 or like a knife-blade through bread, with the least 

 possible friction : and specially is this provision 

 for the lessening of friction important, when we 

 consider that many of these bivalve shell-fish 

 have to move, not only through water, but also 

 through sand and mud. 



It may be assumed that every reader is familiar 

 with the common forms of the bivalve shell-fish. 

 The frontispiece shows one of them, considerably 

 flattened in shape. 



So far, however, we have not explained how 

 animals adapt themselves to circumstances ; we 

 have only pointed out the fact that they do so. 



Take the case of our little Limpet. It cannot 

 say: " I will paint myself with blue and brown, 

 so as to be mistaken for a bit of seaweed reflect- 

 ing the blue sky"; nor can the periwinkle say : 

 "I will paint myself with yellow, so as to pass 

 unnoticed among the yellow ends of the Fucus ; 

 and I will build my spire low, so that it will 

 not be broken." The bivalve shell-fish and the 

 Sand-Cake sea-urchins do not say to one another, 

 " Let us alter our shells, and build them a little 

 flatter, so that we shall not sink in too deep 

 when we lie upon the ooze and sand of the sea." 



How then do these adaptations take place? 

 Darwin has explained this for us. Individuals 

 often have some little peculiarity, in which they 

 differ from the average of their kind; The estab- 

 lishment of such little marks of individuality 



