PROF. T. H. HUXLEY ON THE GENTIANS. 121 
If the facts of distribution of the Gentianem are adequately 
stated in the above propositions, one conclusion appears to me 
to be very clear; and that is, that they are not to be accounted 
for by migration from any "centre of diffusion," to which a 
locality can be assigned in the present condition of the world. 
If the Gentiane of the Andean range had been derived from 
those of Northern Arctogea, or vice versd, it seems unaccountable 
that the proportion of representatives of the types should be so 
different in the two assemblages ; and, still more, that there should 
be only one species common to both. 
There is a well-known case of zoological distribution which 
presents a certain parallelism with that of the Gentiane. The 
genus Tapirus is represented by certain species in South America, 
and by another in the Indo-Malayan region. And paleontology 
furnishes the explanation of the fact by proving the existence of 
Tapirine animals, which have since died out, over the vast inter- 
mediate area in the Middle Tertiary epoch. There has been no 
migration, but simply local modification of the genus at opposite 
ends of its primitively continuous area of distribution, with sub- 
sequent extirpation over the intermediate space. 
lam not aware that any fossil remains of Gentianew have 
been brought to light; but it is known that many of the plante 
of the Middle Tertiary epoch were extremely similar to those 
whieh now exist. Furthermore, it is certain that the great 
elevated areas of both the Old and the New Worlds existed, and 
had attained a considerable height, possibly even greater than that 
which they at present possess, during the Pliocene epoch. Under 
these cireumstances, I see no reason to object to the supposition 
that species of the genus @entiana itself, closely similar to our exist- 
ing species, may have, at that time, occupied pretty much the same 
regions and stations as those in which they are now found, and had 
already adjusted themselves to their hard conditions of existence. 
If so, the changes in the physical geography of the world, which 
preceded and accompanied the Glacial Epoch, must undoubtedly 
have narrowed the range, and probably extirpated a good many 
of the pre-Glacial species of Gentiana. But the remainder would 
have as readily found secure refuge in sheltered nooks and 
valleys, as the existing species do ; and, as the climate improved 
with the passing away of the Glacial epoch, they would start 
afresh from their fastnesses to compete with other immigrants for 
the possession of the new localities laid open to them. 
