196 CONTINUITY OF THE GERM-PIASM AS THE [IV. 



growth in the two series of groups are unequal, and conse- 

 quently an increasing tension is produced which sooner or 

 later, according to the number, arrangement, and energy of 

 the active groups, necessarily renders the continuation of the 

 process impossible. In consequence of such disturbance to 

 the equilibrium, active growth now takes place in the next 

 group, leading to fresh irritation, and this group then reacts 

 more strongly than all the others upon the tension which first 

 stimulated its activity. These changes are repeated until all 

 the groups are gone through, and the ontogenetic development 

 finally reaches the stage at which propagation takes place, and 

 thus the original stage of the germ is reached.' 



Hence, according to Nageli, the different stages of ontogeny 

 arise out of the activities of different parts of the idioplasm : 

 certain groups of micellae in the idioplasm represent the 

 germs (' Anlagen ') of certain structures in the organism : 

 when any such germ reacts under stimulation it produces the 

 corresponding structure. It seems to me that this hypothesis 

 bears some resemblance to Darwnn's theory of pangenesis. 

 I think that Nageli's preformed germs of structures (' Anlagen ') 

 and his groups of such germs are highly elaborated equivalents 

 of the gemmules of pangenesis, which, according to Darwin, 

 manifest activity when their turn comes, or, according to 

 Nageli, when they react under stimulation. When a group of 

 such germs, by their ' active growth' or by their ' irritation, have 

 caused a similar active growth or a similar irritation in the next 

 group, the former may come to rest, or may remain in a state 

 of activity together with its successor, for a longer or shorter 

 period. Its activity may even last for an unlimited time, as 

 is the case in the formation of leafy shoots in many plants.' 



Here, again, we recognize the fact that Nageli's whole 

 hypothesis is intimately connected with the supposition that 

 the entire mass of idioplasm is continuous throughout the 

 organism. Sometimes one part of the idioplasm and some- 

 times another part is irritated, and then produces the corre- 

 sponding organ. But if, on the other hand, the idioplasm does 

 not represent a directly continuous mass, but is composed of 

 thousands of single nucleoplasms which only act together 

 through the medium of their cell-bodies, then we must sub- 

 stitute the conception of ' ontogenetic stages of development 



