DEVELOPMENT THEORY. 83 



But what we have a right to demand of the advocates of 

 development, even now, is that they should show us this 

 tendency ; that they should bring it before us under the form 

 of a slight variation in the anatomical characters of individuals 

 when exposed to certain influences, which continued from 

 generation to generation, would in the end produce the most 

 important modifications in the species. No one denies that 

 the morphological characteristics of individuals are transmitted 

 in different degrees to their descendants. The point which 

 is to be demonstrated is the manner in which an external 

 cause acts in order to impress on the organism the primary 

 modification. Researches of this kind belong to experimental 

 physiology, and this science may even now furnish us with 

 some reliable arguments. 



At the time when Lamarck lived, scientific logic was not 

 very exact in its requirements. In his opinion, a want which 

 was felt, originated the organic conformation suited to satisfy it. 



A certain bird which was in the habit of seeking its food 

 at the bottom of the water, made constant efforts to lengthen 

 its neck, and its nec-k grew longer ; another bird wished to 

 advance as far as possible into the waters of a pond without 

 wetting its plumage ; the efforts which it made to extend its 

 legs gradually gave them, the proportions observed in the 

 wading birds (Grallatores) . The giraffe, attempting to feed 

 on the foliage of trees, gained by this exercise cervical vertebree 

 of a surprising length. 



Lamarck, certainly, attributed to hereditary descent the 

 function of accumulating continually for the profit of the 

 species that which each individual had acquired for his own 

 benefit ; but he did not show what the slight acquisition 

 was which was made by the individual himself, under the 

 influence of external circumstances, and of the habits which 

 he was forced to acquire. J. Hunter reasoned in a similar 

 manner in sciences of a different order. When he wished to 

 explain the cicatrization of wounds and the consolidation 

 of fractured bones, he recognized the necessity that new tissue 

 should be supplied by the blood ; but why did the blood 

 carry these elements to the parts which needed them ? " It 

 was," said he, " in virtue of the stimulus of necessity." 



