THEORIES OF EVOLUTION 137 



acquired characters has not been estabhshed by 

 subsequent discoveries, and his assumption that 

 necessity gives rise to suitable organs is logically 

 untenable unless carefully interpreted. In order to 

 exist, organisms must have everything necessary 

 for existence. A new need may give rise to a new 

 use for some organ already in existence, and so 

 provide a stimulus for additional modification, but 

 if it is need in the absolute sense it must be met 

 at once and not by the development of a new organ. 



Lamarck's first law, that use of an organ 

 strengthens and develops it and that disuse weak- 

 ens and diminishes it, is sufficiently established to 

 be accepted without question. Darwin, no less 

 than his illustrious predecessor, regarded use and 

 disuse as a valid principle in evolution, as well as 

 in the development of individuals, and repeatedly 

 asserted it as a part of his evolutionary theory. 



It is difficult to judge Lamarck's own expression 

 of his theories fairly. There is more than a sug- 

 gestion of truth in them, but the literal meaning 

 of his statements is not one to inspire confidence. 

 When the acceptable features are reduced to the 

 simplest possible form we find in them the idea of 

 use and disuse and the influence of environmental 

 stimuli as the modifying factors in individuals 

 and species. This view necessitates belief in some 

 degree of inheritance of the effects produced in 

 individual organisms. No one now credits the 



