PROBLEM OF DIFFERENTIATION 9 



The Aristotelian view of the causes of epigenesis is complicated 

 and somewhat grotesque from the modern point of view, but it 

 introduces some notions which are very apposite in any discussion 

 of this problem. First of all, Aristotle realised the principle of 

 linked causes, which may be illustrated with reference to the inter- 

 dependence of meshed cogwheels in machinery. He wrote: "that 

 which made the semen sets up the movement in the embryo, and 

 makes the parts of it by having first touched something, though not 

 continuing to touch it".^ This is the principle on which a clock 

 works after it has been wound up, and many thinkers have imagined 

 development as the working of machinery originally wound up and 

 set going at conception, the continued working of which was due 

 to the progressive assumption of causal activity by the results of 

 previous causes. 



But Aristotle did not regard this view as providing a sufficient 

 explanation; in addition, he held that the "soul" was active in 

 controlling the material forces and mechanical processes of de- 

 velopment. Kindred views have been expressed by von Baer and 

 by Driesch. The former held that each stage of development was a 

 necessary conditioji for the production of the following stage, but 

 was not in any full sense its cause, for in addition he regarded the 

 "essential nature" of the parent as responsible for controlling the 

 development of the offspring. Driesch has adapted Aristotle's view 

 of the functions of the "soul" in his theory of entelechies. 



On the other hand, Wilhelm His, having overthrown Haeckel's 

 theory of recapitulation, regarded each stage of development as a 

 sufficient cause of the following stage, and so paved the way for a 

 new branch of science : Entwicklungsmechanik or causal embryo- 

 logy, the foundations of which were laid by Wilhelm Roux. In 

 what may be regarded as the "charter" of the new science, Roux 

 prescribes the analysis of development into so-called complex 

 components, such as assimilation, growth, cell-division, etc. Ulti- 

 mately he supposed these complex components to be reducible to 

 simple components, which in turn would be capable of interpreta- 

 tion in terms of physics and chemistry (Roux, 1885). 



Whether future research will succeed in so reducing the complex 

 components of development as to render them susceptible of ex- 



^ Quoted from Russell, loc, cit. 



