PACKARD. — INHERITANCE OF ACQUIRED CHARACTERS. 335 



animals must be very complex, and at the same time this complexity- 

 must gradually diminish during ontogeny. 



In his latest work Weismanii * thus states his mature and appar- 

 ently final views : "All the phenomena of heredity depend on minute 

 vital units which we have called biophors, and of which living matter 

 is composed ; these are capable of assimilation, growth, and multiplica- 

 tion by division." (p. 450.) He further discusses the nature and mode 

 of action of these hypothetical bodies, which are contained in the nu- 

 cleus, the latter serving as the " bearer of the biophors controlling the 

 character of the cell." How these biophors are grouped into deter- 

 minants, and how the latter form aggregates called ids, the nuclear 

 rods (chromosomes) being aggregates of ids, called idants, is set forth 

 in a very circumstantial way. He then states in the summary of his 

 work : " The germ jrfttsm, or hereditary substance of the Metazoa and 

 Metaphyta, therefore, consists of a larger or smaller number of idants, 

 which in turn are composed of ids ; each id has a definite and special 

 architecture, as it is composed of determinants, each of which plays 

 a perfectly definite part in development." (p. 453.) 



Weisman's reasons for not accepting the doctrine of transmission of 

 acquired characters would appear to be purely hypothetical and a 

 priori, as will be seen by the following extracts : " It is self-evident 

 from the theory of heredity here propounded that only those charac- 

 ters are transmissible which have been controlled — i. e. produced — 

 by determinants of tbe germ, and that consequently only those varia- 

 tions are hereditary which result from the modification of several or 

 many determinants in the germ plasm, and not those which have 

 arisen subsequently in consequence of some influence exerted upon 

 the cells of the body. In other words, it follows from this theory 

 that somatogenic or acquired characters cannot be transmitted. This, 

 however, does not imply that external influences are incapable of pro- 

 ducing hereditary variations; on the contrary, they always give rise to 

 such variations when they are capable of modifying the determinants 

 of tbe germ plasm. Climatic influences, for example, may very well 

 produce permanent variations, by slowly causing gradually increasing 

 alterations to occur in certain determinants in the course of genera- 

 tions. An apparent transmission of somatogenic modifications may 

 even take place under certain circumstances, by the climatic influence 

 affecting certain determinants of the germ plasm at the same time, 



* The Germ Plasm. A Theory of Heredity. Translated by W. N. Parker 

 New York, 1893. 



