164 The Geographical Distribution of Animals. — [March, 
But this principle, if sound, must be carried farther and be 
applied to plants also. There are not wanting indications that 
this may be successfully done ; and it seems not improbable that 
` the reason why botanists have hitherto failed to determine, with 
any unanimity, which are the most natural phytological regions, 
and to work out any connected theory of the migrations of 
plants is, because they have not been furnished with the clue to 
the past changes of the great land masses, which could only be 
arrived at by such an examination of the past and present dis- 
tribution of the higher animals as has been here attempted. The 
difficulties in the way of the study of the distribution of plants, 
from this point of view, will be undoubtedly very great, owing 
to the unusual facilities for distribution many of them possess 
and the absence of any group which might take the place of the 
Mammalia among animals and serve as a guide and standard for 
the rest. We cannot expect the regions to be so well defined in 
the case of plants as in that of animals, and there are sure to 
be many anomalies and discrepancies, which will require long 
study to unravel. The six great regions here adopted are, 
however, as a whole, very well characterized by their vegetable 
forms. The floras of tropical America, of Australia, of South 
Africa, and of Indo-Malaya stand out with as much individuality 
as do the faunas, while the plants of the Palearctic and Neare” 
tic regions exhibit resemblances and diversities of a character 
not unlike those found among the animals. 
This is not a mere question of applying to the vegetable king- 
dom a series of arbitrary divisions of the earth, which have been 
found useful to zodlogists, for it really involves a fundamental 
problem in the theory of evolution. The question we have to 
answer is, firstly, whether the distribution of plants is like that 
of animals, mainly and primarily dependent on the past revo- 
lutions of the earth’s surface, or whether other and altogether 
distinct causes have had a preponderating influence in determin- 
ing the range and limits of vegetable forms; and secondly, 
whether those revolutions have been in their general ouh 
correctly interpreted by means of a study of the distribution an 
affinities of the higher animals. The first question is one for 
botanists alone to answer, but on the second point, the author 
ventures to hope for an affirmative reply, from such of his K = 
ers as will weigh carefully the facts and arguments he has aae 
duced. 
The hypothetical view as to the more recent of the great 
