IV.] FOUNDATION OF A THEORY OF HEREDITY. 187 



formed, and I have defended this opinion for some years past', 

 although I have hitherto laid especial stress on the positive 

 aspect of the question, viz. on the continuity of the germ-plasm. 

 I have attempted to prove that the germ-cells of an organism 

 derive their essential nature from the fact that the germ-plasm 

 of each generation is carried over into that which succeeds it : 

 and 1 have tried to show that during the development of an 

 ^g% into an animal, a part of the germ-substance— although 

 only a minute part— passes over unchanged into the organism 

 which is undergoing development, and that this part represents 

 the basis from which future germ-cells arise. In this way it is 

 to a certain extent possible to conceive how it is that the 

 complex molecular structure of the germ-plasm can be retained 

 unchanged, even in its most minute details, through a long 

 series of generations. 



But how would this be possible if the germ-plasm were 

 formed anew in each individual by the transformation of 

 somatic idio-plasm .? And yet if we reject the 'continuity of 

 the germ-plasm ' we are compelled to adopt this latter hypo- 

 thesis concerning its origin. It is the hypothesis adopted b}' 

 Strasburger, and we have therefore to consider how the subject 

 presents itself from his point of view. 



I entirely agree with Strasburger when he says, 'The 

 specific qualities of organisms are based upon nuclei ;' and 1 

 further agree with him in many of his ideas as to the relation 

 between the nucleus and cell-body: 'Molecular stimuli proceed 

 from the nucleus into the surrounding cytoplasm ; stimuli 

 which, on the one hand, control the phenomena of assimilation 

 in the cell, and, on the other hand, give to the growth of the 

 cytoplasm, which depends upon nutrition, a certain character 

 peculiar to the species.' 'The nutritive cj'toplasm assimilates, 

 while the nucleus controls the assimilation, and hence the 

 substances assimilated possess a certain constitution and 

 nourish in a certain manner the cyto-idioplasm and the nuclear 

 idioplasm. In this way the cytoplasm takes part in the 

 phenomena of construction, upon which the specific form of 

 the organism depends. This constructive activity of the cyto- 

 idioplasm depends upon tlic regulative influence of the nuclei." 



^ This opinion was first expressed in my lecture, ' Ueber die Dauer des 

 Lebens,' Jena, 1882, translated as the first essay in the present volume. 



