ADAPTATION OF TISSUES AND THEIR PRODUCTS 467 



certain tissues or organs within an organism, because they differ from 

 substances produced by other related tissues in the same organism. 



The term "specific" in this sense may refer to a relation between two 

 kinds of substances, or between a substance and an organ or tissue, or 

 between the function and structure of various organs within the same 

 organism, or between an organ or tissue and environmental factors. Such 

 specific relations exist, for instance, between an enzyme and its substratum, 

 between a hormone and the organ on which it acts, between an environ- 

 mental factor and a particular sense organ, and between various organs and 

 organ systems within the same organism. We have here to deal with intra- 

 organismal tissue, organ and substance adaptations. Specific in this sense are 

 also the relations obtaining in general between parasites and hosts, as well 

 as the relations between man and domesticated animals. These may also 

 depend on tissue or organ specificities but in these adaptations there may 

 participate, secondarily, also the organismal differentials, and these specifici- 

 ties are therefore organismal in character and may be classed with type 2. 

 In general if these organ characteristics are specific of individuals or species 

 they become organismal specificities, and such organ or tissue specificities 

 characteristic of species are used largely in determining the systematic posi- 

 tion of plants and animals. 



2. The term "specific" may be used in order to express the fact that a 

 certain structure, substance or function is limited to and characteristic of a 

 certain class or species of organisms, or a certain individual. This is 

 organismal specificity. There is no reference made, in this case to a particular 

 adaptation which this structure, substance or function may bear to others in 

 the same organism. Thus, in certain tropical nymphaeaceae, the pollen-tube 

 grows out and fertilization can occur if the surrounding medium contains a 

 very small amount of boric acid. This is apparently a specific characteristic 

 of these plants and is not known to apply to other plants. In this sense the 

 chitinous integument is specific for certain classes of animals. Limulus and 

 other arthropods have respiratory blood pigments, which are peculiar to 

 these types of organisms. In the metabolism of birds, allantoin plays a specific 

 role. Different hemoglobins are specific for species, and in certain respects, 

 for individuals. We have referred to other similar examples of this kind of 

 specificity in the preceding chapter. While these specific structures, substances 

 or functions may actually enter into relationship with others bearing a corres- 

 ponding organismal differential, we leave this possibility out of consideration 

 under present conditions. A certain combination of structural, metabolic and 

 functional peculiarities is characteristic of a given individual or species. Also, 

 in the realm of psychical-nervous functions there exist specificities of a 

 similar kind. Thus a certain event calls forth in one individual, but not in 

 another, a peculiar reaction, often depending upon a preceding experience of 

 the first individual, which was peculiar to him and not shared by certain other 

 individuals. While these two types of specificity represent distinct character- 

 istics of organs and tissues, still they appear as a rule associated with each 

 other. 



