558 • ON INCREASE IN COMPLEXITY [pt. iii 



place at all, i.e. how from moment to moment the level of actual 

 organisation in the embryo can rise, a question of much theoretical 

 importance to which I shall return in the Epilegomena. But 

 chemical embryology will never allow itself to be restricted to the 

 description of relatively superficial events in the life of the embryo, 

 such as the appearance of enzymes in the digestive tract. Ii will 

 insist on expanding physics and chemistry, if necessary, to cover 

 the animal level of organisation. It will affirm that if we 

 knew all that there is to be known about the physico-chemical 

 constitution of the egg, we should be able to predict the results 

 of its development. This affirmation does not imply that we 

 should be able to make such predictions before any given case had 

 been observed ; for the fact of emergence is real and it is true that 

 knowing all the properties of simpler systems, including how they 

 could combine with one another, does not tell us how in point of 

 fact they actually do combine. The schemes of science are resultant, 

 not emergent, they cannot describe the complex systems before they 

 have been observed, but they can and do offer reasonable causal 

 explanations of them with reference to their simpler factors, after 

 they have been observed. In this way we are not sure that physico- 

 chemical embryology will ever be able to say what a hitherto un- 

 known egg will develop into, but we do expect it some day to be 

 in a position to offer a reasonable causal explanation of the origin 

 of all measurable properties of adult living beings from the measur- 

 able properties of their eggs. And form is evidently one of these, 

 just as is physical constitution. 



There is a great deal of confusion at the present time about such 

 questions and very few workers stop to ask themselves what is the 

 true aim of their studies in causal or exact biology. Moreover, many 

 biologists are uncertain as to the meaning of the facts with which 

 this book is dealing. "I have not so far been able to discern", writes 

 a Belgian embryologist to me, " the truly explicative value of quantita- 

 tive measures in embryology. Far be it from me to minimise the 

 interest of the analysis of metabolism in development, but I have the 

 impression that it is more thrilling for the biochemist than for the 

 embryologist. For the latter the cardinal problems remain the 

 setting-in-action of development, differentiation, or heredity, and 

 their mystery lies wholly in the laws of cell-life. Now these laws 

 have not so far been elucidated, and we hardly know anything of 



