1875.] The History of our Earth. 319 
understood on the supposition of such changes of sea level, 
that the animal population of two neighbouring countries 
separated by the sea is more evidently unlike in proportion 
as such sea is deeper. No probable alteration of sea level 
is likely to have established a bridge between Africa and 
Madagascar. Consequently we discern in their mammals, 
and even in their insects, the most remarkable discrepancy. 
Not one of the monkeys, the cats, or the antelopes, in which 
Africa is so rich, has been found in its island neighbour. 
Both territories are rich in Cetonias, but the species of 
South-Eastern Africa are not the same as those found on 
the opposite shore of the Channel of Mozambique. 
One of the most interesting facts in animal geography is 
the existence, in southern temperate regions, of forms which 
belong to the northern temperate, or even the subar¢tic 
zone. Conversely a few species from extreme southern 
regions have reached northern temperate climates. The 
question has therefore been asked by Mr. Belt and others* 
how the plants and animals of temperate or ar¢tic climates 
could cross the Equator in their migrations from one hemi- 
sphere to the other? ‘The alternate occurrence of a glacial 
period in either hemisphere solves the problem, as was, in 
fact, pointed out by Mr. Darwin. Suppose a cold period 
prevailing in the northern hemisphere and a warm one in 
the southern, both at their maximum intensity. The 
thermal equator, instead of, as at present, approximately 
coinciding with the geographical equator, would be driven 
southwards to—or perhaps beyond—the tropic of Capricorn, 
whilst the northern half of the torrid zone would experience 
a temperate, or perhaps even arctic climate. That such a 
state ‘of things has actually prevailed in inter-tropical 
regions, and that indubitable marks of glaciation exist at 
comparatively low levels in Central America, has been fully 
shown by Mr. Belt. The animals and plants of northern 
extra-tropical regions would then easily manage to reach 
the geographical equator. When, again, the glacial period 
of the northern hemisphere began to relax and the thermal 
Equator returned gradually towards its normal position, 
these organic forms would take refuge on the mountains, 
thence to re-descend and continue their southward journey, 
when the region of greatest heat had passed to the north- 
wards. Fora more detailed exposition of this process the 
reader may consult Darwin’s ‘‘ Origin of Species” (sixth 
edition, p. 339). 
* QUARTERLY JOURNAL OF SCIENCE, O€., 1874. 
VOL. V. (N.S.) 25 
