2o6 Darwin, and after Darwin. 



composed of "bad or even worse than bad species," 

 is of utilitarian significance. 



Be it noted, however, that I am not now ex- 

 pressing my own opinion. There are weighty reasons 

 against thus identifying climatic variations with 

 good species — reasons which will be dealt with 

 in the next chapter. Kerner does not seem to 

 appreciate the weight of these reasons, and therefore 

 I do not call him as a witness to the subject as 

 a whole ; but only to that part of it which has to do 

 with fhe great and general importance of climatic 

 variability in relation to diagnostic work. And thus 

 far his testimony is fully corroborated by every other 

 botanist who has ever attended to the subject. 

 Therefore it does not seem worth while to quote 

 further authorities in substantiation of this point, such 

 as Gartner, De CandoUe, Nageli, Peter, Jordan, &c. 

 For nowadays no one will dispute the high generality 

 and the frequently great extent of climatic variation 

 where the vegetable kingdom is concerned. Indeed, 

 it may fairly be doubted whether there is any one 

 species of plant, whose distribution exposes it to any 

 considerable differences in its external conditions of 

 life, which does not present more or less considerable 

 differences as to its characters in different parts of its 

 range. The principal causes of such climatic variation 

 appear to be the chemical, and, still more, the 

 mechanical nature of soil ; temperature ; intensity and 

 diurnal duration of light in spring and summer; 

 moisture ; presence of certain salts in the air and soil 

 of marine plants, or of plants growing near mineral 

 springs ; and sundry other circumstances of a more 

 or less unknown character. 



