ORGANS AND TISSUES AS CRITERIA 463 



istics, such as the color of the eye. However, structural and functional sub- 

 divisions in the living adult organism do not need to be sharply separated, 

 but transitional areas may gradually lead from one unit to the adjoining one. 

 And all these organ and tissue units, which make up the mosaic of the 

 organism, are connected into one functionally unified whole by means of 

 hormones, including contact substances, and the nervous system. 



Accompanying the structure of organs and tissues are their functions. 

 As they are actually studied, they are essentially the functions of species 

 and not of individuals; they are therefore those which are shared by the 

 individuals of a species ; they bear the character of species differentials. Of 

 this nature is the tendency to maintain a constant osmotic pressure and fluid 

 content in the bodyfluids, termed by us homoiotonia and homoiohydria, to 

 which might be added homoioproteinemia, the tendency to keep the protein 

 content of the blood constant, and, in general, the condition called by Cannon, 

 homoiostasis, which comprises the sum of all the mechanisms which tend to 

 keep the constitution of the bodyfluids, the milieu interne, within narrow 

 limits constant. However, within these functional mechanisms characteristic 

 of species there are those due to the variations of individuals, of which the 

 species type merely represents the average. In different individuals the func- 

 tions of different organs may show independent primary variations, which 

 secondarily may lead to adjustments which concern the individual as a whole. 

 These individual differences in organ functions, associated as they are with 

 visible or invisible structural differences, may also be used for the character- 

 ization and distinction of individuals. 



Besides regulating function, the hormones present in endocrine organs, 

 and similar substances present in other organs, such as bone marrow, liver 

 and kidney, may, in an organ-specific way, regulate also growth, promoting or 

 inhibiting it, and some of these substances may to some extent control the 

 organ in which they originated. But the organs where the hormones are 

 produced, and the various constituents of the nervous system which are 

 endowed with the function of controlling and coordinating other organs, are 

 merely parts of the mosaic organ system. They function by means of their 

 organ characteristics, and the hormones which they produce are not, as a 

 rule, endowed with the organismal differentials which are however present 

 in these organs. But there are indications that some hormones, as for instance, 

 the gonadotropic hormones of the pituitary gland, possess species or class 

 differentials. This specificity applies presumably only to those hormones 

 which chemically have a more complex structure, and which consist of or 

 are combined with proteins. During ontogenetic development, organizers, 

 which at very early stages may induce the reproduction of approximately the 

 whole embryonal organism, but gradually, with the increasing differentiation 

 and specialization of the parts of the organism, become more specialized, 

 exert a controlling, unifying influence in cooperation with the specific 

 substratum on which they act. However, it will be necessary ultimately to 

 trace backward these specific substrata and organizers to simpler structures 

 which represent the precursors of such specific formations. 



Combined with the mosaic individuality is the system of organismal dif- 



