t 



LAMARCK, DARWIN, AND WEISMANN 21 



ing by the most imperfect or the most simple to conclude its labour in the 

 most perfect, has gradually completed their organization ; and of these 

 animals, while spreading generally in all the habitable regions of the globe, 

 each species has received, under the influence of environment which it has 

 encountered, the habits which we recognize and the modifications in its 

 parts which observation reveals in it. 



All that Nature has caused individuals to acquire or lose by the influ- 

 ences of environment to which they have been long exposed, and conse- 

 quently by the influence of the predominant employment of a certain organ, 

 or by that of the continued lack of use of the same part, — all this Nature 

 conserves by generation to the new individuals which arise, provided that 

 these acquired variations (changements) are common to both sexes, or to 

 those which have produced these new individuals. 



But great changes in environment bring about changes in the habits of 

 animals. Changes in their wants necessarily bring about parallel changes 

 in their habits. If new wants become constant or very lasting, they form 

 new habits, the Aew habits involve the use of new parts, or a different use 

 of old parts, which results finally in the production of new organs and the 

 modification of old ones. 



Darwin's later views concerning variation and heredity, 

 as compared with those of Lamarck, may be briefly stated 

 thus: /-^ 



1. Variation was thought to be due either to the two 

 Lamarckian factors, direct action of the environment and use 

 or disuse, or to other as yet unknown causes, the results of 

 which Darwin refers to as " chance variations." 



2. As regards heredity, Darwin seems to have thought 

 with Lamarck that variations of all sorts are inherited, 

 though some doubtless were inherited more strongly and per- 

 sistently than others. 



Weismann (1834-1914). The first great advance, after 

 Darwin, in our knowledge of variation and heredity was made 

 by Weismann, a German zoologist, who within two years after 

 Darwin's death (viz. in 1883) brought forward a new classifi- 

 cation of variations and a new theory of heredity. 



He showed that some variations are congenital {i. e., are 

 horn with us), are in the blood so to speak, while others are 

 acquired through the action of environment, use or disuse. 

 Regarding acquired characters, he showed that these, in all 

 probability, are not inherited. This was a wholly new idea 

 and called forth a hot debate which has not yet ended, but 



