138 



tendency to change. Although it is not necessarily main- 

 tained that the Australian aboriginal has not changed at all 

 since his first appearance upon the earth, yet the presumption- 

 is that if he has changed, he has changed en bloc, and very 

 slowly. As to forms of isolation, naturally one of the most 

 important is geographical isolation. Australia, as an ethnolo- 

 gical province, appears to have been always secluded to a 

 great extent from the rest of the world, so that facilities for 

 the intermingling of other races were but small. In addi- 

 tion to this, Australia, over a very large area, presents a 

 uniformity of physical conditions of an extent scarcely sur- 

 passed in any other portion of the earth. Prof. Tate, in 

 a paper read before the Australian Assoc. Advance, of 

 Science in 1888, on the Influence of Physiographic Changes in 

 the Distribution of Life in Australia, shows how climatic 

 differences in geological times have profoundly adjected the- 

 flora and fauna of respective portions of Australia. A rain- 

 map attached to his paper shows that a scanty 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 fringe 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 

 divergent evolution of a portion of the Australian aborigines. 

 There exist also no physical barriers to a free intercourse 

 between 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 isolation of considerable importance is sexual isola- 

 tion. This is also called by Romanes physiological isola- 

 tion. By it is meant a degree of infertility between groups 

 of intergenerants which leads to< the extinction of the off- 

 spring. This is what occurs in natural selection, whereby 

 there is only a survival of the fittest, the less fittest to the 

 environment or changed conditions disappearing from the 

 scene. Romanes points out that such natural selection can- 

 not conduce to diversity of type, for it is only the fittest type 

 that survives, and the unfit become isolated from the other 

 by a process of extinction. So that, if natural selection 

 has been at work amongst the aborigines to> fit them for the* 

 various physiographic changes that have occurred from time 

 to time, its effects have all been in the direction of monotypic 

 variation. Another kind of sexual isolation is a certain 

 degree of infertility, which may exist even between different 



