188 ACQUIRED CHARACTERS sec. 



enough the importance of the effect of external influences 

 upon the germ (and upon the offspring). ^ Indeed, according 

 to the explanation I have given of the ultimate causes of 

 sexual combination, fertilisation in general is to be regarded 

 as an instance of external action upon the germ-plasm, al- 

 thougli I maintain the original equivalence of the male and 

 female portion of the germ. The view that fertilisation must 

 be regarded as an instance of external action is expressed 

 also by Virchow in his paper on " Descent and Pathology," 

 and he adds : " In a more rigorous sense it can be described as 

 an acquired modification of the egg-cell." He points out 

 further, that in general very much of what is regarded as 

 internal cause {causa interna), is really external influence 

 {causa externcc). " As a rule, a multicellular organism, w^hen 

 variation takes place, is not altered in all its parts ; usually 

 only a portion of the cells is the seat of change. Upon this 

 portion, or to express it more accurately, on each of the cells 

 affected, the remaining unaffected cells can exert an external 

 influence, and conversely the cells originally unaffected can be 

 influenced by those wdiich are affected as by external agents. 

 The conception of a causa externa applies therefore not merely 

 to those influences which affect the organism from without, 

 but also to those which arise from other cells or inner parts 

 and affect the individual cells either on the surface or in the 

 internal parts of the body. Only those causes which really 

 arise from the mechanism of the cells themselves are true 

 internal causes." Thus, then, correlation also is, in accord- 

 ance with my views, regarded as a phenomenon which has its 

 causes in external influences. Further : *' When an infectious 

 substance is generated in one part of the organism and acts 

 upon another part, it is in relation to the latter just as truly 



^ Cf. on this subject the papers of G. Born : Beitrdge zur Bastardirung 

 zwischen den einheimischen Anurenarten, Archiv. f. Physiologie, Bd. xxxii. p. 

 477 ; and E. Pfliiger u. Smith : Untersuchungen iiber Bastardirung der anureii 

 Batrachier u. die Prinzipien der Zeugung, Tbid. p, 558, 559. 



