108 The Unity of the Organism 



organisms that has been made, biology is indebted to C. M. 

 Child. In two recent volumes he has summed up and sys- 

 tematized the elaborate researches prosecuted by him in this 

 field almost exclusively for fifteen years, and has presented 

 his conclusions more fully than in any of his previous writ- 

 ings. 



The limitation set for the present to the constructive part 

 of our enterprise makes it impossible to do more than touch 

 at a few points the great mass of experimental evidence on 

 which Doctor Child bases his conclusions. Fortunatelv, 

 however, the kernel of the conclusions can be stated rather 

 clearly in a small space. 



Although (surprisingly, it seems to me) Child refers 

 hardly at all to the graded meristic series occurring in na- 

 ture, to which the preceding pages have been devoted, it 

 can hardly be doubted that the phenomena with which he 

 deals, and calls "axial gradients," come under the same head 

 as do those which have been occupying us. The phenomena 

 which in the first instance Child has been concerned with, 

 have been brought to light mainly through studies on re- 

 generation in many lower animals. But the general con- 

 clusions reached are far broader than this ; indeed they ex- 

 tend to well-nigh the whole scope of organic growth, but 

 especiall}'- to growth which involves elongation either of the 

 whole organism or of certain parts of organisms. Thus the 

 head-tail type of individual, whether the body be segmented 

 as in arthropods and in many worms, or unsegmented as in 

 other worms and in molluscs, is perhaps the most striking 

 exemplification of the axial gradations with which Child 

 deals. The following quotation shows the generality with 

 which he views the matter from the ontogenic side: "Gra- 

 dients in rate of cell division, size of cells, condition or 

 amount of protoplasm in the cells, rate of growth, and rate 

 and seqvience of differentiation are very characteristic fea- 

 tures of both animal and plant development. Such gra- 



