THE GERM-PLASM THEOEY 353 



^o^_development, not, however, by refemng only to what was 

 qbservableT THrb^^omgJarbeyond i t; on the one hand takin g _the 

 a ppearance of a homog eneous g^rm- substance for reality, and^on the 

 othe i% assuming a special power, which caused a heterogene ous 

 org anism to aris^ frmn a. hrt m^'geaee ug g e rm r 



W^ _cannot now accept either of these assumptions, for w e know 

 that the germ-substance is not homogeneous, and indeed is not merely 

 a substance but aTIfvang cell^^f complex structure ; and we no longer 

 believe" in a spe"cial vital force, and therefore not in a special 'power 

 of development,' which could only be a modification of the former. 

 We are thus as little able to accept the old epigenesis as the old 

 evolution, and we must establish a theory of Development and 

 Heredity on a new basis. 



What this basis must be is in a general way beyond doubt. 

 Since it is the endeavour of the whole of modern biology to 

 interpret life more and more through the interactions of the physical 

 and chemical forces bound up with matter, development, too, comes 

 within this aim, for development is an expression of life. We seek to 

 understand the mechanism of life, and, as a part of that, the mechanism 

 of development and of heredity which is closely associated with it. 



If we wished to attack the problem of heredity at its roots we 

 should first of all have to try to understand the process of life itself 

 as a series of physico-chemical sequences. Perhaps this will be 

 achieved up to a certain point in the future, but if we were to wait 

 for this we should in the meantime have to abandon all attempts at 

 a theoretical interpretation of the phenomena of development and 

 heredity, and might indeed have to postpone them to the Greek 

 Kalends. That would be as though, in the practice and theory 

 of medicine, all investigation into and speculation regarding disease 

 had to wait until the normal, healthy processes of life were thoroughly 

 understood. In that case we should now know nothing of bacteria 

 diseases and the hundred other acquisitions of pathological science: 

 physiology too would have remained far 1:)ehind its present level if 

 it had lacked the fruitful influence of experience in cases of disease, 

 and the ideas and theories, true and false, which have been based 

 thereon. In the same way we require a theory of development and 

 heredity if we are to penetrate deeper into these phenomena, and 

 must have it in spite of the fact that we are still very far from 

 having a complete causal knowledge of the processes of life. For the 

 raw material of observation, which is to some extent fortuitous, will 

 never bring us any further on; observation must be guided by an 

 idea, and thus directed towards a particular goal. 



