X POLYPLACOPHORA. 



animal by a thin pad of girdle. It is to the direct effect of impacts 

 and strains continually brought to bear upon the growing edges of 

 these plates, that their development is due, in precisely the same 

 manner that the enlarged joints of a laborer's hand are the result of 

 the impacts and strains to which they have been subjected. All 

 Chitons which live in situations exposed to the buffeting of the surf, 

 possess highly-developed insertion-plates, which are, moreover, in 

 nearly every cases, conspicuously corrugated for the more effective 

 grasp of the girdle. Examples are the groups Enoplochiton and 

 Mesotomura on the west coast of South America, Acanthopleura in 

 the West Indies and elsewhere, Liolophura in Australia and Japan, 

 all remarkable for the great development of strong, rough insertion 

 plates, and equally for the very exposed situations in which they 

 live, often subjected to the full force of the surf. It is, of course, the 

 belief of the writer that characters acquired by the action of natural 

 forces, acting upon many generations, become hereditary ; but in 

 this case "natural selection " no doubt has had a certain consider- 

 able effect, although the process has, I believe, been mainly one of 

 selection from definite variations produced by the mechanical causes 

 described above, not selection from indefinite variations in all direc- 

 tions. 



On the other hand, forms living in less exposed stations, such as 

 beneath stones at or below low water, have thin, smooth insertion 

 plates (Ischnochiton, etc) ; and at great depths, where the motion of 

 the water and its power of transporting pebbles or stones is reduced 

 to a minimum, and where therefore the valves of the Chitons are not 

 subject to impacts or strains from without, the species are found to 

 be entirely without insertion plates. This excessively weak organiza- 

 tion has been transmitted unchanged from the Palaeozoic Chitons, 

 all of which lacked insertion-plates ; and it is a significant fact that 

 this antique type has been able to exist to the present time only in 

 deep water, where the forces which I believe to have moulded the 

 modern Chitons do not act, and where competition in the life-struggle 

 is less severe than in the shallows. 



In this connection the case of Plaxiphora (Placophoroptis) atlan- 

 tica should be cited. This species was dredged off New England in 

 122 to 640 fms., depths beyond the limit of the penetration of light, 

 and of course far beyond the reach of appreciable water movement, 

 either by currents or surface disturbances. The conditions therefore 

 demand no stronger apparatus for the attachment of the valves to 



