ox DYXA^riC TXFLrENCP:S IX EVOLTTTIOX. y 



As they grow they become too large to escape by the 

 hole through which they entered. Grow they must but the 

 stone walls of their dwelling permit growth only in certain 

 directions. The collector breaks the rock and finds Kellia, 

 Tapes or RupcUaria with the outward conformation of the an- 

 tecedent borer. Those which refused to conform, if an}^ have 

 died. Here we have a case where characters have been as- 

 sumed under an abnormal stress analogous to a pathologic or 

 traumatic mutilation. The progeny of these nestlers would 

 probably exhibit no traces of their parents' deformity. But 

 the pressure of the physical forces on this progeny would be, 

 though invisible, as constant and effective in its results as the 

 rock seemed to be with the nestlers. These results in propor- 

 tion to their harmony with the environment produce upon the 

 obsen'er the impression which is implied when he speaks of 

 the appearance of such species as " normal." 



In my paper on the hinge of Pelecypods and its develop- 

 ment,^^ I have pointed out a number of the particular ways in 

 which the dynamics of the environment ma^^ act on the char- 

 acters of the hinge and shell of bivalve mollusks. 



In a paper now in preparation for publication I have shown 

 how the initiation and development of the columellar plaits in 

 Valuta, Mitra and other Gastropods, is the necessary mechani- 

 cal result of certain comparatively simple physical conditions ; 

 and that the variations and peculiarities connected with these 

 plaits perfectly harmonize with the results which follow with 

 inorganic material subjected to analogous stresses. 



Attention once directed to this class of influences and their 

 effects and it is certain facts will accumulate not less numerous 



*Am. Journ. of Science, Dec, 18S9, p. 445. 



