\ 



198 THEORETICAL BIOLOGY 



a centrifugal, yet in the last instance it is always an impulse 

 that permits the new mechanism to emerge, for both modes 

 of genesis are based on actions. 



Later on we shall have to inquire how it is that the 

 impulses influence our actions in the manufacture of imple- 

 ments. For the moment we are concerned with their invasion 

 of the germ of living organisms, and there we recognise the 

 presence of a material basis consisting of ferments, which, 

 through their relations to the impulses, become independent 

 factors, and these Johannson has called " genes." As Mendel 

 showed, the genes are interchangeable with other suitable 

 genes. The possibility of this exchange depends on the fact, 

 not proved until much later by Driesch, that in the germ 

 there is no framework for genesis. Mendel's doctrine, when 

 understood in its full significance, refutes any mechanical 

 explanation of the developmental process. And that is why 

 I have called Mendel the discoverer of the impulses. 



MENDEL 



If we wish to understand aright the course that Mendel 

 followed in order to arrive at his discovery of the life-factors 

 in the germ, we must first get quite a clear idea of the " charac- 

 ter," which he made the basis of his considerations. By 

 character we understand in a general way every property of 

 an organism that distinguishes its bearer from others of the 

 same kind. Every absolute or relative property of cells 

 and organs may become a character ; to establish characters, 

 there must always be comparison. Colours, hardness, the 

 shape of the organs or their parts and their position relative 

 to one another may be alike in two plants or animals that we 

 investigate, or they may be different. Only in the latter 

 case do we speak of characters. 



There are individual characters, which distinguish the 



