376 THE INFLUENCE OF LIVING SURROUNDINGS. 



oyes themselves. Here, where the molluscs seem to be exempt 

 from pursuit, the eyes and the weapon would alike be useless, 

 and it is quite intelligible that they should not be developed on 

 the back in these species. It is easy too to understand that 

 they must have degenerated if ancestral Onchidia provided with 

 eyes migrated to these regions, where, in consequence of the 

 absence of the fishes, both the organs for defence and those for 

 warning immediately ceased to be of use. In this way the 

 absence of dorsal eyes in the species living in localities where 

 there were no hostile fishes would seem to be a confirmation of 

 the view suggested : That the eyes of those species furnished 

 with them are of use in the way above described. One single 

 difficulty, however, remains ; the West African Onchidia perhaps 

 have no dorsal eyes, and one single species living in the Western 

 Pacific certainly has none, though it lives associated with those 

 fishes, the hereditary foes of its race. But even this exception 

 may easily be explained by a somewhat closer consideration of 

 the structure of the genus, and of the mode of development of 

 the dorsal eyes. 



In a former chapter I have already pointed oub that every 

 living cell or group of cells must possess every attribute of living 

 protoplasm; they must be able to move or change their form; 

 they must bo capable of assimilation, reproduction, respiration, 

 and secretion, and finally they must be capable of elaborating 

 external impressions, and, if I may say so, of transmitting 

 them to their consciousness. We know, moreover, that a lens 

 has the property of collecting in a focus the different rays which 

 combine to make white light, and hence it follows of course 

 that in every papilla of an animal's skin, that is either spherical 

 or formed on any other regular curve, a similar conver- 

 gence of the rays falling upon it must ensue if these portions of 

 the skin are only sufficiently smooth and transparent. The 

 chemical rays or heat-rays which are thus concentrated by the 

 papilla on any point lying within the skin, must be able to act 

 upon some of the cells they impinge upon differently to others, 

 since the reaction of two contiguous cells must always be 

 slightly different. Thus certain cells will be particularly stimu- 

 lated to an increased exertion of the secretive action which is 



