X POLYPLACOPHORA. 



:il Ity a thin pad of girdle. It is to the direct effect of impacts 

 and - ntinnally l>rought to boar upon the growing edges of 



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

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



:m pacts and strains to which they have been subjected. All 

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

 possess highly-developed in>ert ion-plates, which are, moreover, in 

 nearly every cases. : >u<ly corrugated for the more effective 



grasp of the girdle. Examples are the groups Enoplochiton and 



tomura on the west coast of South America, Acanthopleura in 

 the West In. li<-> and el-ewhere, JJn/ojiluira in Australia and Japan, 

 all rmiarkable.forthe great development of strong, rough insertion 

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

 live, t'ten subjected in 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 

 tural 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 



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

 plates (IMfioeAifoft) 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 



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

 be entirely without insertion plates. This excessively weak organ iza- 

 tion has been transmitted unchanged from the Paleozoic Chitons, 

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

 [Ofl typ,- lias 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 J't'U-l/Jwra (PlacojiJinroj,*!*) ntlnn- 

 tica shoul i Thi> species was dredged off New England in 



ni\\n tins., depths beyond the limit of the penetration of li-ht, 

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



r l>y currents or surface disturbances. The conditions therefore 

 demand no stronger apparatus for the attachment of the valves to 



