FLORA. 231 
to bring under the consideration of my readers. The 
views of Hutton and of Count Saporta, which I have cited 
in connection with my 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. Epacridacee are 
restricted to Australia. The genus Viola has two marked 
centres, one in Kurope and anotherin America. The form 
of the European and American species are quite distinct. 
