128 THE PHILOSOPHY OF BIOLOGY 



of the larva into the perfect animal, we cannot fail to 

 conclude that, beside the individual physico-chemical 

 reactions which proceed, there is also organisation. 

 The elementary processes must be integrated. There 

 must be a due order and succession in them. In 

 studying developmental processes, in considering the 

 developing organism as a whole, we are impressed 

 above all else with the notion that not only do physico- 

 chemical reactions occur, but that these are marshalled 

 into place, so to speak. When we attempt to make a 

 description of this integration of those ultimate pro- 

 cesses which we can describe in terms of physical 

 chemistry, physiology fails us. 'At present," says 

 Morgan, " we cannot see how any known principles of 

 chemistry or of physics can explain the development 

 of a definite form by the organism or by a piece of the 

 organism." It is true that we can attempt to imagine 

 a physico-chemical mechanism which is the organisation 

 of the developing embryo ; but this must be a logically 

 constructed mechanism, not only incapable of expe- 

 rimental verification, but which can also be demon- 

 strated, purely by physical arguments, to be false. 

 This conclusion may, without exaggeration, be said to 

 be that of modern experimental embryology. 



There have always been (in modern times) two 

 views as to the nature of the embryological process : 

 (i) that the egg contained the fully formed organism 

 in a kind of rolled-up condition, and that the process 

 of development consisted merely in the unfolding (evolu- 

 tion) of this embryonic organism, and in the increase 

 in volume of its parts. This was the hypothesis of 

 preformation held in the beginning of embryological 

 science. It involved various consequences : the limi- 

 tation, for instance, of the duration of a species, since 

 each generation of female organisms contained in their 



