CHAPTER VII 



EVOLUTION 



THE study of evolution in the Mollusca is an 

 exceedingly complex one, Their extreme plas- 

 ticity naturally renders them both peculiarly suscep- 

 tible and readily responsive to the operation of the 

 two great factors that govern the lives of all animals 

 — namely, the influence on them of their environ- 

 ment and the necessity laid on one and all of pro- 

 curing food. The molluscan mode of life is, in fact, 

 mainly governed by the combined action of these 

 two controlling influences, and in turn itself becom- 

 ing a potent factor, completes the cycle by reacting 

 on the animal, which is thus impelled, so long as 

 similar conditions hold, yet further along a given 

 line of development. 



Owing, however, to the paucity of stable elements 

 to be acted on, continuous progress in any direction 

 has, despite the antiquity of the race, been slow 

 indeed. The total lack of anything like internal 

 framework has militated against any such wonderful 



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