lxiv | COMMENTARY ON THE 
Descending in the systematic scale to the lowest term of the series, the differences 
between the elements of the two Floras become greater and greater, until genera are 
reached; thus, as Mr. Hemsley shows, only 25 to 26 per cent. of these are common to 
the two regions. As yet data do not suffice to ascertain the exact number of species 
common to India and Mexico, but it may not exceed 600 of the 25,273 which is 
approximately the sum of the species of both Foras. 
It is not my purpose to discuss the nature or origin of the unexpected likenesses and 
expected unlikenesses that Mr. Hemsley has shown to exist between the Floras of 
Mexico and India: of these the former are due to causes which have influenced vege- 
tation as a whole; the latter to more or less local causes. As an illustration of what 
I mean, I would say that the conditions which have resulted in monocotyledons main- 
taining their numerical position of one to four or thereabouts of dicotyledons in the 
globe and in all large areas thereof are, in the present state of science, inscrutable ; but 
that the comparatively low number of Compositze in India is explicable by the intrusion 
into India of the Malayan Flora, which is abnormally deficient in Composite. Not 
that this offers any real solution of this latter phenomenon, which lies much deeper. 
It must be shown whether the intrusive Malayan Flora found in India a Flora already 
deficient in Composite, or whether it prevailed over and displaced the pre-existing 
native Composite ; and it must also be shown why the Malayan Flora is deficient in 
this ubiquitously dominant element of all other floras, whether tropical, temperate, or 
frigid, insular or continental, humid or arid. 
It would be very interesting to know whether any of the larger divisions of the 
animal kingdom present phenomena comparable with those derived from large remote 
botanical areas. It may be supposed that the great unconformity that exists between 
the geographical regions of plants and those of animals, as traced out by the most 
competent of zoologists, and which unconformity is so strongly, and as I think rightly, 
insisted upon by Mr. Hemsley, is opposed to such a parallelism existing ; but I do net 
see the force of this objection if, as I think, the problems presented by these “ Arith- 
metics ” are deeper than those of regional distribution. 
Another point of resemblance between the Floras of Mexico and India is that each 
is botanically as well as geographically a n@ud. The lofty mountains in each are con- 
tinuations from more temperate latitudes in the north, favouring an immigration of 
temperate species which have retained their characters in the higher elevations and 
become modified or been extinguished in the lower. In each Rosacew, oaks, and 
Coniferee of northern forms descend into the tropics, even to 3000 feet elevation, whilst 
palms ascend to 8000; and in each epiphytic orchids abound, ascending in cool tem- 
perate regions to 8000 feet and upwards. 
Perhaps the most striking case of parallelism in the Floras of tropical America and 
Asia, as prominently put forward by Mr. Hemsley, is that in each so many temperate 
types, especially oaks and Pinus proper, are continued far into the tropics, but cease at 
