20 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



organ-forming substance has individual peculiarities which depend upon the 

 presence of special chemical groups or on stereoisomeric groups in these pro- 

 teins, but it was not considered probable that this specificity was based on the 

 existence of giant protein molecules, which Pfliiger, and later Verworn, 

 identified with the living protoplasm. It may be added that Herbst subse- 

 quently attributed individual differences to sidechains or smaller radicles of 

 these complex substances rather than to the characteristic structure of giant 

 protein molecules as a whole. However, Fick did not interpret the individual 

 specificity of an egg as due to a single specific substance, but to the sum of 

 peculiarities in the different organ-forming substances in the egg, which latter 

 had been postulated as early as 1880 by the botanist Sachs. Similarly, the 

 individual plasmas of the spermatozoon and of the unfertilized ovum were 

 held to be united in the fertilized ovum, but this combination was thought to 

 lead not merely to a summation of the properties of these plasmas, but to a 

 new specificity. The individuality of the egg represented thus a mosaic of 

 individual peculiarities in the organ-forming substances, and the individual 

 plasma of Fick is therefore quite distinct from the concept of the individuality 

 differential. In the first place it refers to the constitution of the egg and not 

 to the differentiated and integrated individual, and, insofar, it might corre- 

 spond to precursor substances of the individuality differentials. But it differs 

 from the concept of the individuality differential in that it represents the 

 peculiarities of the mosaic of organs and tissues, or of their precursors, the 

 organ-forming substances of the individual rather than those of a substance 

 which is common to all of these organs and tissues. Subsequently Correns re- 

 stricted the meaning of the "individual plasma" of Fick and substituted for 

 it the specific plasma of pure lines in the sense of Johannsen. However, it 

 is clear that in the higher organisms which propagate by fortuitous cross- 

 fertilization such pure lines do not exist and transplantation experiments 

 indicate that among the higher classes of animals pieces of skin of different 

 individuals differ from one another. As far as we know now, the results of 

 homoiotransplantation are never quite the same as those of autotransplantation. 



We see, then, that Fick's concept of "individual" referred to organ speci- 

 ficity, to inherited peculiarities of organs or their differentials. This appears 

 also to be the concept of G. Jaeger, who many years previously had postu- 

 lated differences in the chemical constitution of individuals belonging to the 

 same species on the basis of differences in scents, which may serve to dis- 

 tinguish individuals and species. This kind of specificity concerns individual 

 differences in certain tissues and not something which is the same in all the 

 tissues of an individual. 



Likewise the term "homology," as used by the comparative anatomist 

 Gegenbaur, expresses the similarities in the phylogenetic evolution of corre- 

 sponding organ systems. Both comparative anatomy and paleontology con- 

 sider the similarity inherent in the organs in different species as an indicator 

 of their phylogenetic relationship, and they trace evolution by means of these 

 homologies found in fossils and in still existing species. In these instances we 

 have again to deal with organ specificities and therefore with something 

 distinct from the organismal differentials. 



