gS HOW ANIMALS DEVELOP 



monious sorting out of a train of trucks, can only 

 be seen if there are a lot of trucks, that is to say, 

 it can only happen to a mass composed of several 

 different regions of tissue. The essence of individua- 

 tion is to combine several different parts harmoni- 

 ously together so as to form a complete organ. If a 

 piece of a neural tube is induced, whether we 

 recognize it as a brain or as part of a spinal column, 

 depends on how the top and sides and bottom are 

 shaped and fitted together. The actual organizers 

 we know of always fit the various parts together as 

 harmoniously as possible. We do not find the walls 

 of a brain fitted together with the top and bottom 

 of a spinal column. This tendency to make a complete 

 unit is the most important characteristic properties 

 of the individuation field, as opposed to the evocator. 

 There is one other possibility which is also well 

 expressed in this analogy. In the real sorting yard, 

 the building up of the right trains is not done by a 

 set of compensating levers, such as we have imagined 

 to account for individuation, but instead it is done 

 by a man sitting in the control-box and using his 

 brains. Many authors, the most famous of whom is 

 Driesch, have suggested that we cannot imagine any 

 material compensating system in the embryo which 

 would be efficient, but must postulate a non- 

 material agency which works just like the man in 

 the control tower. This agency is called an entelechy 

 and is supposed to be responsible for the harmonious 

 development of the different parts of the embryo. 

 It is non-material and cannot be directly discovered 



