HEREDITY AND VARIABILITY IN PLANTS. 215 



hypothetical ones like those of a former time are assumed ; the 

 present diversity implies not only equal but the very same 

 anterior diversity, and so on backwards. Or rather it demands 

 a much greater diversity at the outset than now ; for these 

 aberrant forms are the rare exception, and if due to atavism 

 they imply the loss of the many and the incidental reappear- 

 ance of the few. Else they would be the rule instead of the 

 exception, and atavism would be simply heredity. This comes 

 to the view which Mr. Agassiz strongly maintained, that really 

 there are no varieties, — meaning, we understand, that all the 

 forms are aboriginal, except the transient ones evidently due 

 to circumstances. 



That some variation is atavism is clear enough. This is 

 the natural explanation of the appearance of characters want- 

 ing in the immediate parents but known in their ancestors 

 or presumed ancestors. But the assumption of hypothetical 

 ancestors to account for variation generally is quite another 

 thing. Besides its inutility as an explanation, to which we 

 have adverted, its improbability as an hypothesis is set in a 

 strong light by Naudin's own forcible conception of the nature 

 of heredity. What is heredity ? he asks. In other words, 

 what keeps species so true, offspring like parent, through the 

 long line of generations ? He illustrates hereditary force by 

 comparing its action with that of physical force, in which the 

 movement from one state of equilibrium to another is always 

 that in which there is least resistance. From which it follows 

 that when it has once begun to proceed in a certain course, its 

 tendency to continue in that direction increases, because it 

 facilitates its way as it overcomes obstacles. In other words, 

 this line becomes fixed by habit ; vires acquirit eundo ; the 

 stream deepens its bed by flowing ; and the more remote the 

 commencement of a certain course, the more fixed its direc- 

 tion, and the greater its power of overcoming opposition. 

 The species is kept true in its course by the sum of the hered- 

 ities which press each individual forward in its actual direc- 

 tion. So that, as Naudin remarks, if we could calculate the 

 energy with which millions of ancestors tend to impel the liv- 

 ing representative of the line onward in the same direction, we 



