90 THE GERM- PLASM 



It is impossible, as we have already seen, to obtain a satis- 

 factory answer by means of a calculation. But let us assume 

 for the moment that we possess reliable data as to the number 

 of determinants and the size of an id in a particular species. 

 We will further assume that each determinant is composed of, 

 let us say, fifty biophors, and each biophor of a thousand mole- 

 cules, and that the average diameter of a molecule is oTocKpaith mm. 

 Supposing we found that all these units could not be contained 

 within an id of the size we have assumed, we should be forced 

 to conclude that one or more of these quantities had been over- 

 estimated. This result would not weaken the theory of deter- 

 minants, for minute particles inust exist in the germ-plasm for 

 each hereditary and independently variable part of the body. 

 I therefore consider it fruitless to attempt a more accurate esti- 

 mation of the number of determinants in individual species, and 

 to endeavour to find a support for this fundamental theory by 

 means of such calculations. The theory is correct in any case, 

 although our conception of the structure of the germ-plasm may 

 be very incomplete. 



The object of making the above calculation was simply to 

 arrive at this result. The germ-plasm is an extremely delicately- 

 formed organic structure, — a microcosm in the true sense of 

 the word, — in which each independently variable part present 

 throughout ontogeny is represented by a vital particle, each of 

 which again has its definite inherited position, structure, and 

 rate of increase. A tJieo}y of evolution appears to me to be only 

 possible ill this sense. The constituents of the germ-plasm are 

 not miniatures of the fully-formed parts, or even particles exist- 

 ing solely for the formation of the corresponding parts in the 

 body. But each of these particles (the biophors and deter- 

 minants) has a definite and important share in the preceding 

 stages of development, for it takes part in determining the 

 architecture of each idic stage, and consequently also assists in 

 the further ontogenetic disintegration and distribution of the 

 determinants amongst the subsequent cell-stages. All the more 

 essential differences in the structure of organisms depend on 

 this fact. The determinants are particles on whose nature that 

 of the corresponding parts in the fully-formed body depends, 

 whether the latter consists of a sijigle cell or of several or many 

 cells. The assumption of such particles is inevitable in a theory 

 of hereditv, and it alone necessitates an almost inconceivable 



