242 DARWINISM AND THE PROBLEMS OF LIFE 



at once ; it must have been small at first, so that the 

 animal could not retire into it. Thus the shell was of no 

 use at first, and so the snails in which it first made its 

 appearance could not have been favoured by natural 

 selection. 



Nevertheless, the origin of the snail's shell by natural 

 selection is not so unintelligible at all. It has been said 

 that the ancestors of the snails were animals that 

 migrated from the depths of the sea into the surge-zone. 

 These were at first devoid of shells, and clung to the 

 rocks with their foot. The force of the surge must 

 have destroyed numbers of them, tearing the animals 

 from the rocks and dashing them against the stones. 

 Those had an advantage, therefore, which had some 

 feature that modified the force of the surging masses of 

 water. This was a slimy layer that made the back of the 

 animal smooth, so as to give no point of resistance to the 

 water. All snails do, as a matter of fact, secrete this 

 slime, and we can conceive this faculty to be increased 

 by selection. But selection altered the quality, as well 

 as increased the quantity, of the secretion. It was useful 

 for this to become thicker and thicker, and thus at last 

 the shell was formed, covering the snail like a shield. 

 Each small variation was useful from the first, as we 

 saw, and as the variations affected the whole dorsal 

 surface of the body they led in time to the formation of 

 a structure covering the entire animal. When the snail 

 took to further migration and to the land, other 

 variations would be selected until the snails' house was 

 formed, however small it may have been. 



We must be careful in urging the objection that many 



