PREFACE xi 



out of an appropriate mixture of substances. Investiga- 

 tion taught us that all animals develop from the eggy 

 and that every cell arises from another cell. 



Thus *' omnis cellula e cellula " became doctrine. But 

 it was still assumed that the very first living things of 

 all must have arisen from a primordial flux. In this 

 way the attempt was made to get rid of conformity with 

 plan as a natural factor. 



The primordial flux, which was supposed to have 

 been in existence in remote antiquity, now persisted 

 merely as an idea, and so could neither be proved nor 

 disproved by experiment. Accordingly, we have to 

 find other methods in which to attack the question as to 

 whether in living Nature there are independent factors 

 acting in accordance with plan : we must hearken to 

 Nature while she acts in this way, and to the negative 

 statement we must oppose positive evidence. 



In recent years this evidence has so accumulated that 

 we may regard the question as practically settled. To 

 the phrase ''omnis cellula e cellula" we may add, 

 "everything expressing plan is derived from something 

 else that is also in conformity with plan." 



And this means that a new scaffolding is needed for 

 biology : the old scaffolding, borrowed from chemistry 

 and physics, will suffice no longer. For chemistry 

 and physics do not recognise conformity with plan in 

 Nature. Biology, however, consists in the setting up 

 of a scaffolding of doctrine that takes account of this 

 conformity as the basis of life. 



There is a difficulty in the way of this construction, 

 and it is that the concepts of which the scaffolding is 

 made are not directly accessible, but can be got only 

 through a new interrogation. 



As a rule, the text-books that arrange facts under a 

 definite scheme may be read in any sequence : but with 



