FOUNDATION OF A THEORY OF HEREDITY. 181 



everywhere connected by fine processes (as has been proved in animals 

 by Leydig and Heitzmann, and in plants by various botanists), 

 they do not form a network of idioplasm but of somato-plasm ; a 

 substance which, according 1 to Nageli, stands in marked contrast to 

 idioplasm. Strasburger has indeed already spoken of a ' cyto-idio- 

 plasm,' and it is certainly obvious that the cell-body often possesses 

 a specific character, but we must in all cases assume that such a 

 character is impressed upon it by the influence of the nucleus, or, 

 in other words, that the direction in which the cell-substance is 

 differentiated in the course of development is determined by the 

 quality of its nuclear substance. So far, therefore, the deter- 

 mining- nuclear substance corresponds to the idioplasm alone, 

 while the substance of the cell-body must be identified with the 

 somato-plasm (' Nahrplasma') of Nageli. At all events, in practice, 

 it will be well to restrict the term idioplasm to the regulative 

 nuclear substance alone, if we desire to retain the well-chosen terms 

 of Nageli's theory. 



But the second part of Nageli's theory of the idioplasm is also 

 untenable. It is impossible that this substance can have the same 

 constitution everywhere in the organism and during- every stage 

 of its ontogeny. If this were so, how could the idioplasm effect 

 the great differences which obtain in the formation of the various 

 parts of the organism? In some passages of his work Nageli 

 seems to express the same opinion ; e. g. on page 3 1 he says, ' It 

 would be practicable to regard although only in a metaphorical 

 sense the idioplasms of the different cells of an individual as them- 

 selves different, inasmuch as they possess specific powers of pro- 

 duction : we should thus include among these idioplasms all the con- 

 ditions of the organism which bring about the display of specific ac- 

 tivity on the part of cells.' It can be clearly seen from the passages 

 immediately preceding and succeeding the above-quoted sentence, 

 that Nageli, in speaking of these changes in the idioplasm, does 

 not refer to material, but only to dynamical changes. On page 53 

 he lays special stress upon the statement that ' the idioplasm 

 during its growth retains its specific constitution everywhere 

 throughout the organism,' and it is only ' within these fixed 

 structural limits that it changes its conditions of tension and move- 

 ment, and thus alters the forms of growth and activity which are 

 possible at each time and place ' Against such an interpretation 



