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 : (i) 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 



