182 
tendency to change. Although it is not necessarily main- 
tained that the Australian aboriginal has not changed at all 
13 that if he has chan e changed en bloc, and very 
slowly. As to forms of isolation, I one of the most 
important is geographical isolati stralia, as an ethnolo- 
gical province, appears Eum u en secluded to & 
tion to this, Australia, over a very large area, presents a 
uniformity of physical conditions of an extent nura sur- 
passed in any other portion of the earth. Tate, 
a paper read before the Australian Assoc. aps P an of 
Science in 1888, on the Influence of Physiographic Changes in 
the Distribution of Infe in Australia, shows how climatic 
differences in geological times have dis affected the 
flora and fauna of respective portions of Australia. A rain- 
map attached to his paper shows that a cute rainfall of less 
than 10 inches per annum has been the usual average for 
about half the continent, and less than 20 inches for another 
quarter of it, leaving only a well-watered NS chiefly con- 
fined to the eastern and extreme northern sides. The 
change in climate has, then, affected Australia as a whole 
rather than as a part, and there have not been left any 
isolated regions where a different environment of sufficient in- 
tensity might by a process of natural selection have led to the 
^ed evolution of a portion of the Australian aborigines. 
There exist also no physical barriers to a free intercourse 
bitwein t the various separated portions of the tribes, so that 
in Australia itself there was no isolation of a geographical 
form to foster any tendencies to variability of type. Another 
form of ısolation of considerable importance is sexual isola- 
tion. This is also called by Romanes physiological isola- 
tion. By itis meant a degree of intertility between groups 
of intergenerants which leads to the extinction of the off- 
spring. This is what occurs in natural selection, whereb 
there is only a survival of the fittest, the less fittest to the 
ied PH qais or changed conditions disappearing from the 
Romanes points out that such natural selection can- 
iot V coliifacé to diversity of type, for it ıs only the fittest type 
that survives, and the unfit ee vues from the other 
by a process of qoe ction. t, if natural selection 
has been at work amongst the hate to fit them for the 
variation. Another of sexual isolation is a certain 
degree of infertility, which may exist even between different 
