THE EVOLUTION OF THE MAMMALIA. 35 
those continents which have been most lately connected are those 
which have the most similar faunas, while those that have been 
longest separated are those that differ most. The great zoological 
peculiarities of Australia, for example, are to be explained by its 
long isolation from other regions. 
The influx of a mass of immigrating species into any region 
greatly modifies the conditions of the struggle for existence in 
that region, and it may result in the more or less complete 
destruction of the indigenous types, or the latter may continue to 
hold their own and only a few of the invaders be able to gain a 
foothold. South Africa is believed to be an example of the first 
kind, much the greater part of its present mammalian fauna hav- 
ing been derived from the northern continents. South America 
is an example of the second kind, its mammalian fauna being very 
largely indigenous and the immigrant element less important. In 
that continent the sloths, armadillos, and ant-eaters, and the very 
numerous kinds of porcupine-like rodents are indigenous, while 
the carnivorous animals, the llamas, peccaries, and tapirs, are 
migrants from the north, and reached South America at a rela- 
tively late date. In North America the two elements are quite 
evenly balanced. 
This long introduction on the subject of methods of investiga- 
tion is necessary, in order that the reader may see how extremely 
difficult is the task of reconstructing animal genealogies and why 
such different results should have been reached by different observ- 
ers. Progress seems very slow and laborious, but in reality it is 
encouragingly steady, and twenty-five years ago few would have 
ventured to predict such an immense increase in the amount and 
exactitude of knowledge as has been gained in this last quarter 
of the century. If it seems that undue weight has been given to 
palaeontology, that is simply because that science is less com- 
monly known and appreciated, and because it is precisely the one 
which has the best prospects of rapid advance. 
Naturally, the first question that presents itself in our inquiry is 
that concerning the origin of the Mammalia, as a whole. From 
what group or groups of the lower vertebrates are the mammals 
