DEVELOPMENTAL MECHANICS. 179 



and animal physiology as it is almost exclusively carried on by 

 its representatives at present, under the stress of immediate 

 questions ; alongside of this science which treats of the main- 

 tenance-function of parts already established, usually to the 

 exclusion of the formative functions of maintenance ; alongside 

 of the residual " science of the mere keeping a-going of the living 

 machine,'' whereby the functions most difficult of comprehen- 

 sion, viz., those of the construction, formation, and the main- 

 tenance of that which is formed, remain unheeded and unin- 

 vestisated — the science of the causes of this formative 

 activity constitutes an essentially independent branch. 



Since, however, the performance of a function, even in 

 already developed organs, has 2i formative effect in consequence 

 of ''functional adaptation'' to magnitudes of function which 

 have been increased for a considerable time beyond the com- 

 mon mean, or depressed below it, this doctrine of mere machine- 

 activity is of importance to developmental mechanics, and 

 many of the results of its investigation may be of service to 

 the latter, so that we must also remain in close touch with this 

 kind of physiology. But quite as great or even greater will be 

 the assistance which later on this physiology will receive from 

 an insio-ht into the causes of the formation and maintenance of 

 structure. 



Since in plant life the formative functions greatly predomi- 

 nate over the functions of maintenance (Betriebsfunctionen), 

 owing to the absence of the nervous and muscular systems 

 and sense organs, and since, moreover, plants are more easily 

 accessible to experiment than animal organisms, plant physiol- 

 ogy has been spared the onesidedness which exists in animal 

 physiology ; thanks to the investigations of such men as Julius 

 V. Sachs, Wiesner, Pfeffer, Strasburger, Berthold, de Vries, 

 Voechting, Klebs, and others, it has already become in a great 

 measure developmental mechanics in the full sense of the 

 word, and has far outstripped the developmental mechanics of 

 animal organisms. 



The causal tendency of Phytomorphology was considerably 

 advanced by the fact that plant forms, being fixed to a particu- 

 lar spot and hence much more exposed to external influences 



