December 20, 1912] 



SCIENCE 



857 



stitutes them into a class sliarply separated 

 from differentiated tissue cells. Or they 

 may be potential tissue cells which have 

 not yet undergone their definitive trans- 

 formation. When therefore we say that 

 an animal is composed of organs, the or- 

 gans of tissues, and the tissues of cells, we 

 are not merely proposing a classification 

 for the sake of injecting some order into 

 complex structural data. Clearly, this 

 scheme of organic structure represents sub- 

 stantial existence. 



Our conviction of its reality is corrobo- 

 rated by the facts of development. It is 

 true that ontogeny, like phylogeny, is a 

 process within which at every point there 

 is gradual transition from one form to 

 another. Here again, then, are we not 

 arbitrary in attempting to distinguish or- 

 ganizations of distinctly different grades? 

 No, for there is this profound difference 

 between phylogeny and ontogeny. In 

 phylogeny the intermediate forms to a 

 large extent persist as such, and each inter- 

 mediate individual has precisely the same 

 organic value as any individual of either 

 of the species between which it is inter- 

 mediate. In ontogeny the transitional 

 stage is of relatively brief duration. While 

 in this stage the element has the organic 

 value of that unit of higher order which it 

 is destined to become, and not that of any 

 unit of lower order. It is intermediate 

 therefore only in external aspect. It is po- 

 tentially an element of a distinct type and 

 it is assuming the structural characteristics 

 of that type as rapidly as the organic ener- 

 gies concerned can elaborate them. On- 

 togeny, then, while it is in a sense a process 

 in which there is gradual change from one 

 thing to another, is nevertheless a process 

 .whose essential feature is the establishment 

 of sharply marked differences. This com- 

 parison between phylogeny and ontogeny 



is, of course, open to the objection that we 

 describe the developmental process with 

 reference to its end, which we are able to 

 observe, while the end of the phylogenetic 

 process does not yet appear. Finally, the 

 sequence in which structural systems make 

 their appearance in ontogeny corresponds 

 to the relations which they exhibit in the 

 adult. In general we actually see, in the 

 embryo, cells building up tissues, tissues 

 building up subsidiary organs, and these 

 uniting to form successively higher organic 

 complexes. Were the sequence otherwise, 

 we might well doubt if our conception of 

 organizations of various grades, one sub- 

 ordinated to another, had any real value. 

 When, therefore, we attempt to liken a 

 tissue to a species, the comparison soon 

 becomes forced. It is quite clear that the 

 tissue is a real thing, a definite configura- 

 tion of matter, exhibiting certain physical 

 and physiological properties which can 

 only be regarded as the expression of a pre- 

 cisely corresponding dynamic complex. 

 The species, no less real, is a human con- 

 cept. In view, then, of the known facts of 

 adult structure and of ontogeny, and by 

 comparison of these facts with what we 

 know of phylogeny, we can hardly escape 

 the conclusion that our conception of the 

 individual as representing, in its entirety, 

 the highest of a descending series of or- 

 ganizations is, so far as it goes, a statement 

 of biological truth. 



CONSEQUENCES 



Granting that this conception of the con- 

 stitution of the individual organism repre- 

 sents substantial reality, the problems 

 therein presented to us are not rivaled in 

 importance by any with which biology has 

 to deal. The problems of heredity and 

 evolution are intimately, inseparably, re- 

 lated to this one of organization, for they, 

 too, represent one aspect or another of the 



