No. 507] BOTANICAL SOCIETY OF AMERICA 147 



Lamarck and Saint-Hilaire than lie was himself aware. 

 He thus becomes the connecting link between these two 

 great but less appreciated seers, and the investigators 

 of to-morrow whose success will rest on the experimental 

 study of the habitat as the primary cause. 

 Causes of Variation. 



I have hitherto sometimes spoken as if the variations so common and 

 multiform with organic beings under domestication, and in a lesser 

 degree under nature— were due to chance. This, of course, is a 

 wholly incorrect expression, but it serves to acknowledge plainly our 

 ignorance of the cause of each particular variation. 5 



No doubt each variation must have its efficient cause, but it is as 

 hopeless to discover the cause of each as it is to say why a chill or a 

 poison affects one man differently from another. Even with the modi- 

 fications resulting from the definite action of the conditions of life, when 

 all or nearly all the individuals which have been similarly exposed, are 

 similarly affected, we can rarely see the precise relation between cause 

 and effect. 8 



We must . . . conclude that organic beings, when subjected during 

 several generations to any change whatever in their conditions, tend to 

 vary: the kind of variation which ensues depending in most cases 

 in a far higher degree on the nature or constitution of the being, 

 than on the nature of the changed conditions. 7 



Those authors that adopt the latter view— that variability must be 

 looked at as an ultimate fact, would probably deny that each separate 

 variation has its own exciting cause. Although we can seldom trace 

 the precise relation between cause and effect, yet the considerations 

 presently to be given lead to the conclusion that each modification 

 must have its own distinct cause, and is not the result of what we 

 blindly call accident. 8 



Habitat as Cause. 



With respect to what I have called the indirect action of changed 

 conditions, namely, through the reproductive system being affected, we 

 may infer that variability is thus induced, partly from the fact of this 

 system being extremely sensitive to any change in the conditions. 

 Many facts clearly show how eminently susceptible the reproductive sys- 

 tem is to very slight changes in the surrounding conditions.' 



Changed conditions of life are of the highest importance in causing 

 variability, both by acting directly upon the organization, and indi- 

 rectly by affecting the reproductive system. It is not probable that 



