434 PROGRESS OF SCIENCE IN THE CENTURY. 



mental pattern, it may be, which we feel bound to 

 call original, some structural change which suggests 

 a new departure. We may tentatively interpret this 

 as due to some fresh permutation or combination of 

 the complex nuclear and cellular substances which 

 are mingled at the outset of every new life sexually 

 reproduced. The plausibility of this interpretation 

 is increased when we remember that our inheritance, 

 as Galton has so clearly shown, is mosaic rather than 

 dual. For it is not merely in an intermingling of 

 maternal and paternal contributions that life be- 

 gins, but of legacies through the parents from re- 

 moter ancestors. The complexity of the problem 

 is increased, not diminished, if there be reality in 

 the conception that the different hereditary qualities 

 may have a struggle in nuce, or that there is a ^' ger- 

 minal selection '' as Weismann calls it. 



Another possibility of variation has been sought in 

 the fact that the hereditary material is doubtless 

 very complex and has a complex environment within 

 the parental body. If it has, in spite of its essential 

 stability, a tendency to instability as regards minor 

 details, we may perhaps find the change-exciting 

 stimuli in the ceaseless nutritive oscillations within 

 the body. But enough has been said to indicate 

 how uncertain is the voice of biology in answering 

 the fundamental questions as to the nature and origin 

 of variations. 



Modifications. — Among the observed differences 

 which mark man from man, trout from trout, but- 

 tercup from buttercup, there are many to which we 

 cannot apply the term variations. Quite apart from 

 constitutional or germinal changes there are differ- 

 ences which are obviously impressed upon the body 

 from without, such as sun-burning, or which result 



