OLD AND NEW THEORIES COMPARED. 343 



not apply to a system which maintains that in case an 

 animal feels any given want it will gradually develop 

 the structure which shall meet the want that is to 

 say, if the want be not so great and so sudden as to ex- 

 tinguish the creature to which it has become a necessity. 

 For if there be such a power of self-adaptation as thus 

 supposed, two or more very widely different animals 

 feeling the same kind of want might easily adopt 

 similar means to gratify it, and hence develop even- 

 tually a substantially similar structure ; just as two 

 men, without any kind of concert, have often hit 

 upon like means of compassing the same ends. Mr. 

 Spencer's theory so Mr. Mivart tells us and cer- 

 tainly that of Lamarck, whose disciple Mr. Spencer 

 would appear to be,* admits "a certain peculiar, but 

 limited power of response and adaptation in each ani- 

 mal and plant " to the conditions of their existence. 

 " Such theories," says Mr. Mivart, " have not to con- 

 tend against the difficulty proposed, and it has been 

 urged that even very complex extremely similar struc- 

 tures have again and again been developed quite inde- 

 pendently one of the other, and this because the process 

 has taken place not by merely haphazard, indefinite 

 variations in all directions, but by the concurrence of 

 some other internal natural law or laws co-operating 

 with external influences and with Natural Selection in 

 the evolution of organic forms. 



" It must never le forgotten that to admit any such 

 constant operation of any such unknown natural cause is 

 to deny the purely Darwinian theory which relies upon 



* See ante, p. 330, lino 1 after heading. 



