20 INTRODUCTORY I 



of what is obviously in some sense a predetermined process, 

 without presupposing the preformation in the germ of morpho- 

 logical units representing every possible inheritable character, 

 has issued in Herbst's and Driesch's conception of the events of 

 ontogeny as so many responses to stimuli exerted by the develop- 

 ing parts on one another. At the same time the need for 

 inquiry into the external conditions and the part they may play 

 in growth and differentiation has not been forgotten. 



Thus though it would be vain to pretend that the ideal of 

 a complete causal explanation has yet been realized, still some 

 material has been gathered for an answer to each of the two 

 main questions : what are the internal, what the external con- 

 ditions that determine the course of development ? These 

 questions we shall discuss in the following pages. It will then 

 only remain to inquire whether a causal explanation in the 

 accepted sense of the phrase a mechanics of ontogeny which 

 resolves the single occurrences first into general physiological 

 laws, and these in the last resort into the generalizations of 

 physics and chemistry, can ever afford a theory which may be 

 said to be complete either from a scientific or from a philo- 

 sophical point of view. Should the mechanical explanation 

 prove to be scientifically insufficient, it may be necessary, with 

 Driesch and the neo-vitalists, to invoke a consciousness of the 

 end to be realized to guide and govern the merely material 

 elements ; but even were this not so it would still be incumbent 

 upon us to consider whether the end itself not the conscious- 

 ness of it is not the final, and yet none the less the principal 

 determining cause of the whole process. 



LITERATURE 



ARISTOTLE. De generatione animalium, ed. Bekker, Oxford, 1837. 

 De pavtibus animalium, ed. Bekker, Oxford, 1837. De anima, ed. 

 Trendelenburg, Berlin, 1877. 



C. BONNET. CEuvres, Neuchatel, 1781. Contemplation de la nature, 

 vii me partie ; Considerations sur les corps organises ; Memoire sur les 

 germes ; Palingenesie philosophique. 



G. BORN. Ueber den Einfluss der Schwere auf das Froschei, Arch, 

 mikr. Anat. xxiv, 1885. 



