130 OTHER FACTORS OF EVOLUTION 



mann's doctrine, so as to bring out the real difference between it 

 and that of those who incHne to the Lamarckian view. His 

 doctrines will be found very distinctly stated in one of his " Essays 

 upon Heredity," » though scattered passages in his recent work, 

 " The Evolution Theory " will help to clear up doubtful points. 



New characters in plants or animals are, he holds, either {a) 

 blastogenic and transmissible, or (6) somatogenic and non-trans- 

 missible. Modifications of the latter type due to reaction of the 

 soma under altered external influences are regarded as alterations 

 limited to the individual, because they are supposed to leave the 

 germ-plasm unaffected. 



The new characters, then, that can be transmitted to progeny 

 are those only that are correlated with and due to blastogenic 

 changes (or changes in the germ-plasm). There is, however, 

 nothing distinctive in Weismann's point of view here. It is a doc- 

 trine which is taught also by Herbert Spencer, and by those who 

 agree with him. 



The real difference which divides the two schools is as follows. 

 According to Weismann changes in the soma cannot lead to, 

 or be associated with, definite and correlated changes in the 

 germ-plasm ; though changes in the germ-plasm are in some 

 inscrutable way, through the intermediation of his countless 

 armies of determinants and biophors, supposed by him to be 

 capable of leading to precise correlated changes in the soma. 

 According to Spencer and others, however, changes in the soma 

 induced by external conditions may become associated with 

 definitely related changes in the germ-plasm. Such correlated 

 changes are held by them to be no more impossible or inscru- 

 table than those which we see every day actually occurring 

 in the reverse order, namely in the development of animals and 

 plants from their germs : although in view of the excessively com- 

 plicated nature of the processes, "we are obhged," as Spencer 

 says, " to admit that the actual organising process transcends 

 conception." 



Thus, in the opinion of Weismann, the soma cannot communicate 

 precise changes to the germ-plasm, though the germ-plasm can 

 lead to an exact building-up of the soma ; while according to 

 Spencer the germ-plasm and the soma are mutually capable of 

 influencing one another in specific ways. 



Edition of 1891, pp. 423-428 especially. 



