FLORA. 2S1 



to bring under the consideration of ray readers. The 

 views of Hutton and of Count Saporta, which I have cited 

 in connection with ray reference to the nebular hypothesis 

 of Laplace, I have adduced only as a working hypothesis 

 sufficient to remove objections which might suggest them- 

 selves to my readers, and to make comprehensible the 

 views advanced in regard to the diffusion of vegetation 

 from the polar region towards the equator. 



To make this subject more intelligible, I may further 

 remark : that the existing distribution of vegetables on 

 the earth's surface has been greatly determined by condi- 

 tions of soil and climate favouring or arresting the growth of 

 plants produced by seeds dispersed from some parent plant. 

 Amongst conditions of soil operating thus may be reckoned 

 its constituents, including moisture and the state of dis- 

 entigration in which it exists. Amongst conditions of 

 climate may be reckoned its humidity and temperature, 

 and the maximum, minimum, or medium measures of this. 

 As a result of this we find that there are zones of latitude 

 and of altitude marked by characteristic vegetation, and 

 that there are forms of vegetation which have become 

 characteristic of various localities ; we have the palm form 

 in some, that of the minosa in others, the coniferous form 

 in others, the eucalyptous form in others, and the heath 

 form in others. Professor J. H. Balfour, in a chapter on 

 Geographical Botany in his volume entitled Outlines of 

 Botany, says : 



' We sometimes meet with marked centres, where the 

 maxima of the genus of an order, or of the species of a 

 genus occur, the number of the genera or species diminish- 

 ing as we recede from these centres, and ending perhaps in 

 a solitary representative in some distant country. Gentians 

 and Saxifrages have their maxima in the European Alps; 

 Erocaulons have their great centre in Brazil, but a few 

 species are found in other countries. EpacridaceaB are 

 restricted to Australia. The genus Viola has two marked 

 centres, one in Europe and another in America. The form 

 of the European and American species are quite distinct. 



