272 C. M. CHILD 



or not. We can only say that they seem to be wholes. But we 

 can see very clearly that such wholes are not alike and we shall 

 find that in Planaria their unlikeness is, at least in certain respects, 

 not a matter of 'chance' but is, subject to certain laws. The 

 point where 'wholeness' ceases to exist is not a definite point 

 determined in nature, but an arbitrary convention of the human 

 mind. Cases of resemblance to the original animal or to the 

 'norm' within certain limits we distinguish as wholes from cases 

 where the resemblance is less. In short we have here a distinc- 

 tion of the same kind as that existing between 'normal' and 

 'abnormal.' The normal is merely the usual under the usual 

 conditions. It is not at all difficult to distinguish between a 

 'normal' or a 'whole' planarian on the one hand and a tailless 

 head or a headless tail on the other, but between these different 

 extreme forms we find many intermediate gradations and doubt- 

 less many others exist which we cannot distinguish. We cannot 

 determine where the organism begins to become headless or tail- 

 less or in general abnormal, because this point is a human con- 

 vention not a datum of nature. 



And finally attention may be called to the fact that the experi- 

 ments on form regulation afford facts and conclusions of great 

 interest for the general problem of physiological correlation : they 

 enable us to learn more of the physiological character of organic 

 individuality than is at present possible in any other way. 



The present paper is concerned primarily with the demonstra- 

 tion of, first, the existence of certain differences in constitution 

 along the chief axis of the planarian body, and second, the exist- 

 ence of physiologic correlation between these regions of differ- 

 ent constitution. The method of investigation is the compari- 

 son of pieces of the same size from different regions and of pieces 

 of different size from as nearly as possible the same regions. 

 What I have termed the constitutional and correlative factors 

 in the regulation of pieces are in other words the factors of size 

 (length) of the piece and region of the body from which it is taken. 



