I a THEORY OF ORGANIC EVOLUTION. 



combined into more or less complex arrangements, 

 so that the cross section of the strand represents 

 the configuration of the idioplasm. * 



Each ontogeny (individual) begins in a minute 

 germ cell, in which a small quantity of idioplasm is 

 contained. In the cell divisions, by which the 

 organism grows, the idioplasm divides into as many 

 parts as there are single cells, while it continually 

 increases in quantity in a corresponding degree. 

 The ontogenetic increase of the idioplasm takes 

 place by length growth of the strands that is, by 

 intercalation of micellae in each row of cells of the 

 strands, which thereby grow in length without 

 changing the configuration of the cross section.! 



* Nageli makes his idioplasm ramify throughout the organism in un- 

 broken continuity, much like a system of nerves in the higher animals. 

 This idea with Nageli was purely speculative. It was known that the pro- 

 toplasm is in connection throughout the organism, but it has been proved 

 more recently that only the somatic protoplasm is thus connected. The 

 part in which the essential nature of the organism is contained is localized 

 in the nucleus and hence might properly be designated as nucleoplasm, as 

 Weismann suggests. If the idioplasm is localized in the nucleus, it cannot 

 be continuous throughout the system, as Nageli assumes. But this objec- 

 tion applies only to a detail of the theory and does not affect the fundamen- 

 tal conception, that of a portion of the protoplasm which is differentiated 

 from the rest and represents a definite molecular structure which deter- 

 mines the specific nature of the organism. Tram. 



t Hence, according to Nageli, every cell of the organism has idioplasm 

 of identical structure. This at once suggests the objection, how can the 

 idioplasm, for instance, of a pollen grain be the same as that of a leaf? 

 Identical idioplasms should always produce identical structures, Nageli 

 attempts to explain this difficulty by attributing the different results to dif- 

 ferent "conditions of tension and movement," i.e a dynamical difference 

 between the idioplasms of the different parts of the organism. (Abstam- 

 mungslehre, p. 53.) 



This idea of differences of structure being due to dynamic rather than 

 to material causes plays a considerable part in Nageli's theory, but is the 

 point on which he speaks with least certainty in fact with a noticeable 

 hesitation. He does not clearly explain the phrase " conditions of tension 



