weismann's tiikory of heredity. 441 



ditioiis of life. In other words, so far as the iini-celhilar orj^anisms are 

 concerned, Weismaun is rif>idly and exclusively an advocate of the 

 theory of Lamarck, just as much as in the case of all the multi cellular 

 organisms he is rigidly and exclusively au opponent of that theory. 

 Nevertheless, there is here no inconsistency; on the contrary, it is con- 

 sistency with the logical requirements of his theory that leads to this 

 sharp partitioning of the uni-cellular from the multi-cellular organisms 

 with respect to the causes of their evolution. For, as he points out, 

 the conditious of propagation among the uni-cellular organisms are such 

 that parent and offspring are one and the same thing ; " the child is a 

 part, and usually a half, of its ])arent." Therefore, if the parent has 

 been in any way modified by the action of external conditions, it is in- 

 evitable that the child should, from the moment of its birth (i. e., fissi- 

 ])arous sejiaration). be similarly modified; and if the modifying intlu- 

 ences continue in the same lines for a sufficient length of time the re- 

 sulting change of type may become sulEciently pronounced to consti- 

 tute a new species, genus, etc. But in the case of the multi-cellular or 

 sexual organisms the child is not thus merely a severed moiety of its 

 parent ; it is the result of the fusion of two highly specialized and ex- 

 tremely minute particles of each of two parents. Therefore, whatever 

 may be thought touching the validity of Weismann's deduction that 

 in no case can any modification induced by external conditions on these 

 parents be transmitted to their progeny, at least we must recognize the 

 validity of the distinction which he draws between the facility with 

 which such transmission must take place in the uni cellular organisms 

 as compared with tlie difficulty — or, as he believes, the impossibility — 

 of its doing so in the multi-cellular. 



We are now in a position to fully understand Professor Weismann's 

 theory of heredity in all its bearings. Briefly stated, this theory is as 

 follows : The whole organization of any multi-cellular organism is com- 

 ])Osed of two entirely different kinds of cells, namely, the germ cells, 

 or those which have to do with reproduction, and the somatic cells, or 

 those which go to constitute all the other parts of the organism. Now 

 the somatic cells in their aggregations as tissues aiul organs may be 

 modified in numberless ways by the direct action of the environment 

 as well as by special habits formed during the individual life-time of 

 the organism. But although the modifications thus induced may be 

 and generally are adaptive, — such as the increased muscularity caused 

 by the use of muscles, " practice makiug perfect " in the case of nervous 

 adjustments, and so on, — in no case can these so-called acquired or 

 " somato-genetic " characters exercise any influence upon the germ-cells, 

 such that they should re-appear in their products (progeny) as congen- 

 ital or '^ blasto-genetic" characters. For according to the theory, the 

 germ-colls as to their germinal contents differ in kind from the somatic 

 cells, and have no other connection or dei)endence u[)on them than 

 that of deriving from them their food and lodging. Ho much then for 



