10 THE THEORY OF [pt. 



"the autonomy of biology". This precludes the promise of an ever- 

 increasing homogeneity in the structure of science, and hence an 

 ever-increasing simplicity. 



The Historical Perspective 



That the older embryologists awaited the extension of physico- 

 chemical conceptions to embryology is no mere matter of conjecture. 

 Until the mechanical theory of the universe had been consolidated 

 by the " corpuscularian philosophy" of the seventeenth century it 

 would be useless to look for illustration of this, but by 1674 John 

 Mayow was tracing the part played by the " nitro-aerial particles" 

 in the development of the embryo, and in 1732 Hermann Boerhaave 

 was discussing chemical problems with explicit reference to embryonic 

 development. Many other examples of this point of view in the 

 eighteenth century will be given later. Then, when the second decade 

 of the nineteenth century had nearly gone, von Baer, perhaps the 

 greatest of all embryologists, was careful to preface his Entwicklungs- 

 geschichte by a careful account of all that was known about the 

 chemical constitution of the Qgg, and that, although his philosophical 

 inclinations were deeply vitalistic, and even his practical interests 

 morphological. In Roux, of course, this future reference came out 

 explicitly, and the extension of biochemistry into embryology was 

 allowed for and foreseen. An early instance was the association be- 

 tween Wilhelm His and Hans Miescher. Miescher, writing to Hoppe- 

 Seyler in 1872 said, "I am now collecting material from fishes, 

 birds, and amphibia to lead to a chemical statics of development. 

 With this end in view I shall do analyses of ash, nuclein, and lecithin". 



Embryology before Harvey, however, was rigidly Aristotelian, a 

 statement the meaning of which George Santayana has lucidly ex- 

 plained. "Aristotle", said he, "distinguished four principles in the 

 understanding of Nature. The ignorant think that these are all, 

 equally, forces producing change, and the cooperative sources of all 

 natural things. Thus, if a chicken is hatched, they say that the Efficient 

 Cause is the warmth of the brooding hen, yet this heat would not 

 have hatched a chicken out of a stone, so that a second condition, 

 which they call the Material Cause, must be invoked as well, namely, 

 the nature of an egg; the essence of eggness being precisely a capacity 

 to be hatched when warmed gently — because, as they wisely observe, 

 boiling would drive away all potentiality of hatching. Yet, as they 



