FACTOES OF INHERITANCE. 57 



of results. So far we are presented with a picture of 

 independent particles without any clue as to how they 

 co-operate or succeed in producing the finished result. 

 Students of genetics are apt to overrate the importance 

 of an explanation of the mere mechanism of transmission. 

 Not until the factors have been brought into relation 

 with the general metabolism, with growth and repro- 

 duction, will the theory of heredity approach com- 

 pletion. 



It is not possible as yet to describe scientifically the 

 process of embryonic development, but some sort of 

 epigenesig there must be. All we need assume is that 

 there are factors enough in the inheritance to produce a 

 certain definite result. Whe;i, under the same conditions, 

 the results come to differ, when a mutation arises, no 

 doubt some corresponding change must have taken place 

 in the inheritance, some alteration in the composition and 

 properties of what we vaguely call the factors (p. 52). 

 A consideration of the changes undergone by complex 

 chemical compounds may give us some notion of what 

 the factorial changes may be. The student of Organic 

 Chemistry is familiar with long series of closely allied 

 compounds, formed of molecules often containing vast 

 numbers of atoms. The properties of these compounds 

 all differ from one another, owing to the elements not 

 being the same, or to their not being present in exactly 

 the same proportions, or again to their being differently 

 grouped within the molecule. The properties may be 

 altered at will by removing this or that atom, by intro- 

 ducing a new element, by substituting one group for 

 another, or by merely shifting the grouping within the 

 molecule. The resulting changes will be definite, will 

 appear large or small, important or not, according as they 

 strike our senses. In some such fashion do we imagine 

 the changes occur in the factors of inheritance. 



Mutations are generally said to arise spontaneously ; 

 this, of course, is only another way of saying that we do 



