Characters as Adaptive and Specific. 207 



Before closing these remarks on climatic variation in 

 the vegetable kingdom, prominent attention must be 

 directed to a fact of broad generality and, in relation 

 to our present subject, of considerable importance. 

 This is that the same external causes very frequently 

 produce the same effects in the way of specific change 

 throughout large numbers of tmrelated species — i. e. 

 species belonging to different genera, families, and 

 orders. Moreover, throughout all these unrelated 

 species, we can frequently trace a uniform correlation 

 between the degrees of change and the degrees to 

 which they have been subjected to the causes in 

 question. 



As examples, all botanists who have attended to 

 the subject are struck by the similarity of variation 

 presented by different species growing on the same 

 soils, altitudes, latitudes, longitudes, and so forth. 

 Plants growing on chalky soils, when compared with 

 those growing on richer soils, are often more thickly 

 covered with down, which is usually of a white or 

 grey colour. Their leaves are frequently of a bluish- 

 green tint, more deeply cut, and less veined, while 

 their flowers tend to be larger and of a lighter 

 tint. There are similarly constant differences in 

 other respects in varieties growing on sundry other 

 kinds of soils. Sea-salt has the general effect, on 

 many different kinds of plants, of producing moist 

 fleshy leaves, and red tints. Experiments in trans- 

 plantation have shown that these changes may be 

 induced artificially ; so there can be no doubt as to its 

 being this that and the other set of external conditions 

 which produces them in nature. Again, dampness 

 causes leaves to become smoother, greener, less cut, 



