CHAPTER VII 
EVOLUTION 
HE 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, howevet, 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 
too 
