486 ZOOLOGY SINCE DARWIN. 



confounding of cause and condition contrary to the ordinary use of the 

 terms. 



Systematic zoology and the comparative anatomy of living beings 

 show us that the possibilities of development for germs are almost 

 infinitely varied, and though any particular germ can only pass over 

 the same path as did the innumerable generations of its ancestors, this 

 can only be due to internal causes; that path must have been there 

 ready to be traversed; that is to say, there must be fixed external 

 circumstances (conditions) present if the causes are to make their 

 operation manifest. 



E. Haeckel has said l that with the inheritance of acquired peculiari- 

 ties the entire theory of descent must stand or fall. This would only 

 be correct if there were no other explanation for the variability of 

 organisms. But Weismann does not deny that the conditions of exist- 

 ence may have an alterative influence upon the constitution of the germ- 

 plasm, and during long spaces of time many components of the conge- 

 ries of forces belonging to the latter may increase in power, others 

 decrease; indeed even new sources of force may be introduced. But 

 these operations are direct and not caused by the action of any of the 

 other organs of the body. 



To support the hypothesis of such a direct influencing of the germ- 

 plasm by external agents, we are forced to refer to those organisms 

 that are propagated asexually (monogenically), for in germs which are 

 produced bisexually (digenically) there are already possible, because of 

 the intermixture of hereditary material from two different parents, an 

 extraordinary number of combinations in the conqDosition of the germ- 

 plasm, so many that it is probable that the separation of a portion of 

 the nucleus (the polar bodies), which is observed to occur before the 

 beginning of segmentation, is an operation for the elimination of a 

 superfluous number of hereditary tendencies. 2 



The cause of variation lies, therefore, in the individual changes that 

 occur in the composition of the germ-plasm, and what we call "adapta- 

 tion," is not an active and immediate achievement of a single indi- 

 vidual, but the result of a more or less complicated process of selection 

 that extends throughout generations of individuals. The external 

 conditions, to whose influence animals respond in a definite manner 

 peculiar and appropriate for each species, are therefore in no way the 

 true causes of such reactions, but merely release a morphogeuic force 

 that already resides in the germ and can only be put in action by just 

 these precise circumstances. 



If we accept the view here presented, there will be noted a remark- 

 able contrast between the germ cells and the " soma," that multiform 



E. Haeckel, Zur Phylogenie tier australischen Fauna; Systematische Einleitung. 

 In R. Semon, Zoologische Forsckungsreisen in Australian und dem malayischen 

 Archipel, I. Bd., Jena, 1887. 



2 A. Weismann, Uber die Zahl der Richtuugskoiper und liber ihre Bedeutung fiir 

 die Vererbung. Jena, 1887. 



