256 Darwin, and after Darwin. 



effort by intelligence. Its explanatory power in the 

 case of most invertebrata as well as in that of all plants 

 is extremely limited, inasmuch as these organisms 

 can never be moved to a greater or less use of their 

 several parts by any discriminating volition, such as 

 that which leads to the continued straining of a 

 giraffe's neck for the purpose of reaching foliage. In 

 the second place, even among the higher animals there 

 are numberless tissues and organs which unques- 

 tionably present a high degree of adaptive evolution, 

 but which nevertheless cannot be supposed to have 

 fallen within the influence of Lamarckian principles. 

 Of such are the shells of Crustacea, tortoises, &c., 

 which although undoubtedly of great use to the 

 animals presenting them, cannot ever have been used 

 in the sense required by Lamarck's hypothesis, i. e. 

 actively exercised, so as to increase a flow of nutrition 

 to the part. Lastly, in the third place, the validity of 

 Lamarck's hypothesis in any case whatsoever has of 

 late years become a matter of serious question, as will 

 be fully shown and discussed in the next volume. 

 Meanwhile it is enough to observe that, on account of 

 all these reasons, the theory of Lamarck, even if it be 

 supposed to present any truth at all, is clearly in- 

 sufficient as a full or complete theory of organic 

 evolution. 



In historical order the next theory that was arrived 

 at was the theory of natural selection, simultaneously 

 published by Darwin and Wallace on July ist, 1858. 



If we may estimate the importance of an idea by 

 the change of thought which it effects, this idea of 

 natural selection is unquestionably the most important 



