Chap. XXIII. CONDITIONS OF LIFE. 281 



state of nature may be modified in various definite ways by 

 the conditions to which, they have been long exposed, as in 

 the case of the birds and other animals in the northern and 

 southern United States, and of American trees in comparison 

 with their representatives in Europe. But in many cases it 

 is most difficult to distinguish between the definite result of 

 changed conditions, and the accumulation through natural 

 selection of indefinite variations which have prove serviceable. 

 If it profited a plant to inhabit a humid instead of an arid 

 station, a fitting change in its constitution might possibly 

 result from the direct action of the environment, though we 

 have no grounds for believing that variations of the right 

 kind would occur more frequently with plants inhabiting a 

 station a little more humid than usual, than with other plants. 

 "Whether the station was unusually dry or humid, variations 

 adapting the plant in a slight degree for directly opposite 

 habits of life would occasionally arise, as we have good reason 

 to believe from what we actually see in other cases. 



The organisation or constitution of the being which is 

 acted on, is generally a much more important element than 

 the nature of the changed conditions, in determining the 

 nature of the variation. We have evidence of this in the 

 appearance of nearly similar modifications under different 

 conditions, and of different modifications under apparently 

 nearly the same conditions. We have still better evidence of 

 this in closely parallel varieties being frequently produced 

 from distinct races, or even distinct species ; and in the 

 frequent recurrence of the same monstrosity in the same 

 species. We have also seen that the degree to which 

 domesticated birds have varied, does not stand in any close 

 relation with the amount of change to which they have been 

 subjected. 



To recur once again to bud-variations. When we reflect on 

 the millions of buds which many trees have produced, before 

 some one bud has varied, we are lost in wonder as to what 

 the precise cause of each variation can be. Let us recall the 

 case given by Andrew Knight of the forty-year-old tree of the 

 yellow magnum bonum plum, an old variety which has been 

 propagated by grafts on various stocks for a very long period 



