ADAPTATION OF TISSUES AND THEIR PRODUCTS 469 



tween organisms graded in accordance with their relationship depended on 

 substances which were not identical with the primary organismal and individu- 

 ality differentials. We have encountered reactions of this latter kind for instance 

 in transplantations among embryos which do not possess organismal differen- 

 tials in the strict sense, but instead possess precursors of these differentials ; we 

 have encountered examples of this kind also among unicellular organisms and 

 among algae; but there is reason for assuming that also in other cases the 

 equilibrium between the parts of an organism and its graded interaction with 

 other organisms may depend on substances other than the typical fully 

 developed organismal differentials. We have seen that the characteristics of 

 certain organs and tissues may also be used in the classification of organisms 

 and that the development of the organs and tissues and their differentials 

 from simple structures and substances to more complex, differentiated ones 

 has taken place in association with the corresponding development of the 

 organismal differentials. Substances other than organismal differentials may 

 be involved in the reactions which exhibit specific adaptations between organ- 

 isms. Some of the substances which are the bearers of these specifically 

 adapted relations seem to be relatively simple, heat resistant substances, of 

 neither a protein nor of a complex carbohydrate or lipoid nature, therefore 

 quite distinct from the organismal differential substances in the strict sense, 

 although the possibility exists that they are derived from the latter type of 

 differentials. Our present limited knowledge does not make it possible, in 

 many instances, to distinguish between these different types of substances and 

 the specific reactions which they cause. We may then apply in these cases the 

 term organismal differentials in a wider sense, which includes substances 

 which are concerned with the production of specific adaptations. 



We may now cite some examples of specific adaptations in the interaction 

 of preformed substances which may or may not carry organismal differentials. 

 It can be shown that there exist in the blood sera of various classes or species 

 of vertebrates, substances which in combination with certain other substances, 

 the tissue coagulins present in tissue extracts may cause either an acceleration 

 or an inhibition of blood coagulation, in accordance with the kind of animals 

 from which the sera or extracts were obtained, and in accordance with the 

 length of time during which these two kinds of substances were allowed to 

 act on each other before they were added to the blood plasma which served 

 as test material. Now, there is evidence that the substances in tissue extracts 

 and sera which act together or perhaps combine to form agents accelerating 

 the coagulation of the blood are specifically adapted to each other, and there 

 is likewise a probability that also the inhibiting substances are, in the same 

 sense, specifically adapted to each other. This would mean that the accelerat- 

 ing, and perhaps also the inhibiting, precursor substances in serum and 

 extract carry class or species differentials, and that when substances carry- 

 ing the same or related differentials interact, the effect on coagulation is greater 

 than when substances carrying disharmonious differentials interact. A specific 

 adaptation is also noticed between tissue extract and blood plasma, the tissue 

 extract of the same class as that from which the plasma has been obtained, 



