208 Darwin, and after Darwin. 



and the flowers to become darker ; while dryness 

 tends to produce opposite effects. I need not go on 

 to specify the particular results on all kinds of plants 

 of altitude, latitude, longitude, and so forth. For we 

 are concerned only with the fact that these two 

 correlations may be regarded as general laws apper- 

 taining to the vegetable kingdom namely, (A) that 

 the same external causes produce similar varietal 

 effects in numerous unallied species of plants ; and, 

 (B) that the more these species are exposed to such 

 causes the greater is the amount of varietal effect 

 produced so that, for instance, on travelling from 

 latitude to latitude, longitude to longitude, altitude 

 to altitude, &c., we may see greater and greater 

 degrees of such definite and more or less common 

 varietal changes affecting the unallied species in 

 question. Now these general laws are of importance 

 for us, because they prove unequivocally that it is the 

 direct action of external conditions of life which 

 produce climatic variations of specific types. And, 

 taken in connexion with the results of experiments in 

 transplantation (which in a single generation may 

 yield variations similar to those found in nature under 

 similar circumstances), these general laws still further 

 indicate that climatic variations are " indifferent " 

 variations. In other words, we find that changes of 

 specific characters are of widespread occurrence in the 

 vegetable kingdom, that they are constantly and even 

 proportionally related todefiniteexternal circumstances, 

 but yet that, in as far as they are climatic, they can- 

 not be attributed to the agency of natural selection 1 . 



1 Since the above paragraphs hare been in type, the Rev. G. Henslow 

 has published his Linnaean Society papers which are mentioned in the 



