1896. ZOOLOGY SINCE DARWIN. 315 



E. Haeckel has asserted' that the whole theory of descent stands 

 or falls with the inheritance of acquired characters. This would be 

 so only if there were no other explanation for the variability of the 

 organism. However, even Weismann does not deny that the condi- 

 tions of existence exercise an influence over the constitution of the 

 germ-plasm, and may, during long periods, strengthen many compo- 

 nents in the combination of forces of the latter, weaken others, and 

 even introduce new sources of energy. But these influences will only 

 act directly, and not through the medium of other organs of the body. 



In order to accept this direct influence of the germ-plasm by 

 external agents, we are specially directed to those organisms with 

 monogonic reproduction. In amphigonic organisms, the simple mix- 

 ture of the inheritance substance of two parent-individuals produces 

 an extraordinary number of possible combinations in the composition 

 of the germ-plasm — so many that the extrusion (observable before 

 the beginning of development) of a part of the nucleus of the ovum 

 (the so-called polar body) was believed to indicate an arrangement for 

 the elimination of the superfluous inheritance-tendencies. ^ 



The cause of variation lies then in the individually changing 

 composition of the germ-plasm, and what we call "adaptation" is no 

 active and direct performance of the individual, but is the result of a 

 more or less complicated selective process working through generations 

 of individuals. The external conditions, to whose influence animals 

 respond in exact, regulated ways, peculiar to each species, are there- 

 fore in no wise the real causes of such reaction, but release a formative 

 energy, latent in the germ, and brought into action only under these 

 definite conditions. 



If we accept the view here set forth, there results a noteworthy 

 contrast between the germ-cells and the " soma " — that multiform and 

 often extraordinarily complicated combination of the remaining organs 

 of the body. Principally in this, that in the segmentation that takes 

 place during the development of the egg, those cells which in the 

 perfected organism will play the role of germ-cells, receive a portion 

 of all those parts of the egg which are necessary for the re-formation 

 of a complete individual ; while, in the soma-cells, only those sub- 

 stances necessary to the formation of a particular organ or organ - 

 complex are transmitted. At the same time, however, the soma 

 becomes a kind of organ of the reproductive cells, which accomplishes 

 the work of assimilation, moves, feels, and thinks — but thereby serves 

 in some respects only the development of the germ-cells, and so 

 ensures the continuity of life. 



{To be continued.) 



^ E. Haeckel, " Zur Phylogenie der australischen Fauna. Systematiscbe 

 Einleitung." In R. Semon, " Zoologische Forschungsreisen in Australien und dem 

 malayischen Archipel," i. Bd., Jena, 1893. 



2 A. Weismann, " Uber die Zahl der Richtungskorper und iiber ihre Bedeutung 

 fiir die Vererbung." Jena, 1887. 



z 2 



