LIMITING FACTORS IN THE HISTORY OF SCIENCE 



Morphological embryology, Physiological embryology, and Chemical 

 embryology form to-day a vast range of factual knowledge, without 

 any great unifying hypothesis, for we cannot as yet dignify the 

 axial gradient doctrines, the field theories, and the speculations on 

 the genetic control of enzymes, with such a position. We cannot 

 doubt that the most urgent need of modern embryology is a series 

 of advances of a purely theoretical, even mathematico-logical, nature. 

 Only by something of this kind can we redress the balance which 

 has fallen over to observation and experiment; only by some such 

 effort can we obtain a theoretical embryology suited in magnitude 

 and spaciousness to the wealth of facts which contemporary investi- 

 gators are accumulating day by day. 



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