No. 627] 



ADAPTATION 



351 



eration show that there must be smaller bodies within the 

 nucleus, each containing the potentialities of the entire 

 organism. Ritter^^'' has recently insisted that the con- 

 cept of heredity must be applied unreservedly to these 

 one-celled organisms, many of which are quite complex 

 in structure and undergo a true ontogeny. Indeed, the 

 experimental studies of Jennings and his students have 

 demonstrated the transmission of individual peculiar- 

 ities, both of structure and function. As for the metazoa, 

 despite the considerable evidence for chromosomal "indi- 

 viduality" and for the localization of genetic ''factors," 

 it seems to be entirely premature for us to assume the 

 existence of a mosaic of parts, rigidly predetermined and 

 incapable of making good a loss. One should recall what 

 happened to an earlier ''mosaic theory" of development. 



To go to the other extreme, it might be supposed that 

 for each form of organism there was at least one sub- 

 stance, or molecular structure, which was typical for it, 

 and which determined its specific physical and chemical 

 characteristics. The other constituents of the adult body 

 would be modifications of this typical substance, which 

 had lost certain of its original components or acquired 

 new ones. This specific protoplasm would have some 

 points in common with the "germ plasm" of Weismann. 

 It might be credited with the ])ow('r of iiule(iiut(' growth 

 and self -division, so loiiu' ;»> these \vci-i> not rlu'cktMl l)y 

 counterbalancing forces, ^^'hvn coiupletely dierked, a 

 growth equilibrium would be established which would 

 represent the uoniial form of the si)ecies in ([uestion. 



The rather vaii'ue and iuik-liuite ])oiut of view here sug- 

 gested would avoid, lu)\vever, the tau-le of unverified 

 assumptions that are involved in the hypothesis of a 

 "germ-plasm," conceived as an aggregation either of 

 Weismannian "determinants" or twentieth-century 

 "genes." The admitted possibility that certain mate- 

 rial particles of the nucleus are functionally related to 

 separately heritable adult characters does not constitute 



21b The Unity of the Organism, Chapt. XII, XIII. 



