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28 THE WORLD OF LIFE CHAP.
former. The variety of species is small in arctic or sub-arctic
lands, where the long and severe winter allows of only certain
forms of vegetable and animal life ; and it is equally if not
more limited in those desert regions caused by the scarcity
or almost complete
absence of streams and of rain. It is
most luxuriant and most varied in that portion of the tropics
where the temperature is high and uniform and the supply of
moisture large and constant, conditions which are found at
their maximum in the Equatorial Zone within twelve or fifteen
degrees on each side of the equator, but sometimes extending
to beyond the northern tropic, as on the flanks of the
Himalayas in north-eastern India, where the monsoon winds
carry so much moisture from the heated Indian Ocean as to
produce forests of tropical luxuriance in latitudes where most
other parts of the world are more or less arid, and very often
absolute deserts.
Temperate Floras compared
I will now endeavour to compare some of the chief floras
of the Temperate Zone, both as regards the total number of
species in fairly comparable areas, and the slight but clearly
marked increase of the number in more southern as compared
with more northern latitudes.
I will first show how this law applies even in the com-
paratively slight difference of latitude and climate within our
own country. Dividing Great Britain (without Wales) * into
three nearly equal portions Scotland north of the Forth and
Clyde, Mid -Britain, and South Britain, including all the
southern counties; with areas of 22,000, 26,000, and 31,000
thenumber of species (in 1 8 70) was, respectively,
squaremiles
930, 1148, and 1230. At the same period the total of
Great Britain was 1425 species. These figures are all
obtained from
must therefore
Mr. H. C. Watson's Cybele Britannica,
be considered to be fairly comparable.
and
We
see here that the whole of the Scottish Highlands, with their
rich alpine and sub-alpine flora, together with that of the
sheltered valleys, lakes, and mountainous islands of the
west coast, is yet decidedly less rich in species than Mid-
1 Wales is omitted in order to make the three divisions more equal, and con-
trasted in latitude only.