2Q4 C. M. CHILD. 



is theoretically possible, provided certain characteristic mechan- 

 ical conditions exist in the normal animal and are altered in char- 

 acteristic manner in the piece. In several of the "Studies" the 

 changes in shape in pieces of Stenostoma, Leptoplaua and Cesto- 

 plana were shown to be apparently primarily mechanical regula- 

 tions (Child, 02, '04*7, 'O5<2, '05^) and it was suggested that 

 similar processes might occur elsewhere. The recognition of 

 mechanical regulation is not in any sense an attempt to interpret 

 regulation in general on a mechanical basis (cf. Child, '02, p. 

 229) : it concerns merely changes in shape and outline. Me- 

 chanical regulations are possible only under the conditions men- 

 tioned above, and whether they occur or not can only be deter- 

 mined by investigation of each particular case. It is not necessary 

 to suppose that all cases of " morphallaxis " (in the sense of change 

 in shape and proportions) are mechanical regulations. If they are 

 they certainly depend on different mechanical conditions in dif- 

 ferent cases. In Hydra, for example, the factors determining the 

 change in shape are certainly not the same as in Planaria, and 

 may not be mechanical at all. In the cases which I have con- 

 sidered mechanical strains arise in connection with locomotion 

 and these strains are altered in a characteristic manner in iso- 

 lated pieces and, as I have shown, the changes in shape are 

 exactly what might be expected in physically plastic material 

 under these conditions. Mechanical regulation is to be expected 

 only in organisms or parts possessing a considerable degree of 

 physical plasticity and where skeletal structures are not con- 

 cerned. As regards the shape of animals in general gross 

 mechanical factors are certainly unimportant as compared with 

 others, or not concerned at all, and I have never even suggested 

 that such factors were of general significance (cf. Driesch, '05, p. 

 790). On the other hand, it seems to me absurd to deny that 

 characteristic strains or pressures existing in plastic material may 

 play some part in determining shape. 



That the changes in shape in pieces of Stenostoma, Leptoplana, 

 Ccstoplana, and Planaria are not primarily " functional adapta- 

 tions " (cf. Driesch, '05, p. 766) is, I think, evident. A func- 

 tional adaptation, as I understand it, is a change in structure 

 which involves a change in functional capacity, either an increase 



