406 TRANSMISSION 



Weismann's exact words concerning the two varieties of 

 butterflies the darker Italian and the lighter German are 

 as follows : l " The two varieties may have originated owing to 

 a gradual cumulative influence of the climate, the slight effects 

 of one summer or winter having been transmitted and added to 

 from generation to generation"; and again, 2 " In many other 

 animals and plants influences of temperature and environment 

 may very possibly produce permanent hereditary variations in a 

 similar manner " ; and still again, 3 " Many varieties of plants 

 may also be due wholly or in part to the simultaneous variation 

 of corresponding determinants in some part of the soma and in 

 the germ plasm of the reproductive cells, and these variations 

 must be hereditary. Temperature, and nutrition in its widest 

 sense, affect the whole body of the plant the somatic cells as 

 well as the germ cells." 4 



In all this it must be noted that Weismann limits action of 

 this kind to such external influences as are able to penetrate the 

 organism and affect the germ plasm directly ; whether an influ- 

 ence does this is to be determined in every case by direct ex- 

 periment, and is not to be assumed from the effect of the same 

 influence upon mere body development. 



These citations from Weismann, while not especially bearing 

 upon the topic of use and disuse, are introduced because his 

 position as the leader in opposition to the theory of the trans- 

 mission of modifications due to external influences is often mis- 

 understood. They are introduced at this point because it is 

 concerning use and disuse that the most vigorous discussions 

 have arisen. 



Those believing in the transmitted effects of use and disuse 

 base their belief mainly upon the method of proof by instance, 

 and most of them cite instances that were far better omitted. 

 As long as one side depends upon simple enumeration, and 

 the other mainly upon abstract reasoning, we are not likely to 

 get ahead. As all forms of acquired characters are discussed 

 together, it is practically impossible to cite references dealing 

 exclusively with use and disuse ; but in order that the student 



1 Weismann, Germ Plasm, p. 400-406. 8 Ibid. p. 406. 



2 Ibid. p. 405. * Italics are mine. 



