16 INTRODUCTION 
could be passed on by inheritance from parent to off- 
spring, and so accumulated from generation to genera- 
tion. In the case of animals Lamarck conceived the 
production of a new specific form to take place in the 
following way : Owing to some change of external con- 
ditions, the desire to perform some new kind of action 
was set up in the parent species, and by the hereditary 
effect of the striving occasioned by this desire a modi- 
fication of the organs affected into forms better fitted 
to carry out the new function was gradually achieved. 
Thus Lamarck supposed that snakes were evolved 
from a pre-existing type of animal which was of a much 
less attenuated shape, and which possessed two pairs 
of limbs like any other vertebrates. And he supposed 
this evolution to have taken place owing to the con- 
stant striving of these animals to pass through narrow 
crevices ; the effect of such striving being inherited, and 
so accumulated from one generation to another. 
In the case of plants, in which conscious effort is 
precluded, a similar result was supposed to have been 
attained by an hereditary accumulation of the effects 
of the environment. 
2. The explanation of Darwin, or at least the Neo- 
Darwinian form of it, as interpreted by Wallace, 
Weismann, and others, and as opposed to and exclud- 
ing the view of Lamarck, was as follows : Two separate 
factors are primarily concerned : (1) the fact of fluc- 
tuating variation—the fact that no two members of 
the same family ever resemble one another exactly ; 
and (2) the occurrence of a struggle for existence 
between organisms owing to the geometric rate of 
