ON THE CAUSES OF VARIATION. 809 



ment, passes away, the better for all concerned. It is one more of 

 th-ose wonders of which Byron wrote : 



"Thus saith the preacher, 'Naught beneath the sun 

 Is new,' yet still from change to change we run ; 

 "What varied wonders tempt us as they pass ! 

 The cow-pox, tractors, galvanism, and gas 

 In turns appear, to make the vulgar stare. 

 Till the swollen bubble bursts — and all is air ! " 



— English Bards and Scotch Reviewers. 



ON THE CAUSES OF VARIATION.* 



By C. V. RILEY, Ph. D., Ukited States Entomologist. 

 II. — Cwicluded. 



HAVING thus summarily indicated those factors of evolution 

 associated with genesis and which are essentially physio- 

 logical, however much psychical phenomena may co-operate, we 

 may touch upon the more purely psychical factors or those per- 

 taining to the growth and use of mind, employing the term to 

 express those neural phenomena traceable to the medium of the 

 brain. Their importance in evolution increases with increasing 

 cephalization and complexity of nerve system. For the present 

 purpose, however, it is with the objective side of psychology, or 

 what may be called psycho-physiology, that we must deal. 



PsycJiical — Use and Disuse. — Full consideration of the effect 

 of use and disuse involves a discussion, not only of the question 

 of the transmission of acquired structures, but of the influence of 

 individual effort and of necessity — i. e., a considerlation of .tn^ 

 essentially Lamarckian factors in evolution. \.The occasion wiH 

 not permit me to do full justice to these subjects. That /unction- 

 ally produced modifications are inherited -^as the grea't assump- 

 tion upon which Lamar^ founded his theory of evolution. Majiy 

 able naturalists have insisted on it, and in my judgment th^re 

 should no longer be any doubt whatever of the fact, not only so 

 far as grosser structure is concerned, but brain-structure likewise. 

 No question is of more moment in the whole range of biology, and 

 especially biologic philosophy, and Spencer has well pointed out 

 that on the answer to it will depend largely the sciences of psy- 

 chology, ethics, and sociology. Weismann, Lankester, and others 

 deny hereditary power in such modifications, the former believ- 

 ing that hereditary modification can result only from changes in 

 the germ plasma, i. e., are virtually congenital. Natural selection, 

 according to this view, plays upon the germ plasma ; but I have 



* From the address of the Vice-President of Section F of the American Association for 

 the Advancement of Science, delivered at the Cleveland meeting, August, 1888. 



