396 HISTORY OF THEORIES 



differ from ordinary cells ? to what do they owe their unique 

 reproductive power ? In short, what enables them to develop 

 into organisms like the parent-organisms ? To these questions 

 it is possible to give a satisfactory answer. 



The Architecture of Inheritance. — The second problem is 

 of a different nature, and much more difficult. In some way, 

 every one must admit, the germ-cells or gametes are potential 

 organisms. Without any aid except that afforded by an 

 appropriate environment, they can develop into complete 

 organisms. In some way, the organism, the inheritance, lies 

 in posse in the germ-cells. Can we form any image of this ? 

 Can we construct any hypothetical scheme of the manner in which 

 the inheritance is organised within the germ-cells ? Chemists 

 frame hypothetical conceptions regarding the structure of 

 chemical molecules, and judge of the validity of these by their 

 usefulness in formulating the changes which the molecules under- 

 go in certain conditions ; physicists make similar mental pictures 

 — imaginary models — of the constitution of atoms and so on. 

 Can biologists do the same in regard to the material basis of 

 inheritance ? 



This is the fundamental problem of inheritance, and it can 

 only be approached indirectly. The organisation can never be 

 seen or verified ; all the complexities in germ-cells which micro- 

 scopic analysis reveals are not more than the rough outlines of 

 the real edifice — the edifice which the scientific imagination 

 must build. But the speculative construction is not left to 

 irresponsible fancy ; it must be such that it corresponds to and 

 enables us to formulate the visible and measurable facts of 

 inheritance, and the processes of development. It must be 

 harmonious with the large generalisations of inheritance, such 

 as Mendel's law or Galton's law ; it must also be harmonious 

 with every peculiar phenomenon, such as resemblance to a 

 remote ancestor. 



Theory of Development. — A careful study of the history 



