DEVELOPMENTAL MECHANICS. 159 



- 

 II. Methods of Investigation in Developmental Mechanics. 



The causal method of investigation, /car efo^i/, is experi- 



ment. This statement holds good of the mechanics of develop- 

 ment more than of any other line of causal investigation, as 

 will be apparent from the following considerations : 



The formative operations occurring in the organism are 

 hidden from sight ; we cannot see the ganglion cells of the an- 

 terior cornua influencing the development of the muscles, nor 

 increased activity stimulating the growth of organs, nor the sub- 

 stances secreted by cells exerting a chemiotropic attraction on 

 other cells ; indeed, it is not even possible to observe directly 

 that pressure is exerted by cells during growth, nor the passive 

 alterations in the form of parts on which such pressure is 

 exerted. All these operations can only be inferred. 



The ascertainment of these operant conditions is, moreover, 

 made still more difficult because the really formative activity is 

 carried on so rapidly, as compared with any visible changes, 

 that even in the production of considerable transformations the 

 efficient causes, the antecedent is, according to His (9), almost 

 always in advance of the effect, or consequence, by a differential ; 

 even in eventually resulting passive deformations the nature of 

 the processes cannot be ascertained by removing the pressing 

 parts because the form resulting from the pressure has in every 

 case already settled into internal equilibrium and lacks only a 

 minimum of adaptation ; for after the removal of the press- 

 ing parts a passively deformed structure does not return to its 

 original form, as does a bent rubber tube after the cessation of 

 the bending forces. 



Since, moreover, during the normal development of an indi- 

 vidual there are always many changes taking place simultane- 

 ously, we can only conclude from observation of these changes 

 that the ensemble of former changes is or may be the cause of 

 the changes which follow ; but we are not in a position to con- 

 clude on what preceding change each single ultimate change 

 depends. 



In accordance with the aphorism : two phenomena which 

 always occur together are causally connected, we can, it is 



