174 ZOOLOGICAL GEOGRAPHY. [PART Ill. 
South America, Australia, and South Africa with Madagascar— 
have been more or less completely isolated, during long periods, 
both from the northern continent and from each other. These 
latter countries have, however, been subject to more or less im- 
migration from the north during rare epochs of approximation 
to, or partial union with it. In the northern, more extensive, and 
probably more ancient land, the process of development has 
been more rapid, and has resulted in more varied and higher 
types; while the southern lands, for the most part, seem to have 
produced numerous diverging modifications of the lower grades 
of organization, the original types of which they derived either 
from the north, or from some of the ancient continents in Meso- 
zoic or Palozoic times. Hence those curious resemblances in 
the fauna of South America, Australia, and, to a less extent, 
Madagascar, which have led to a somewhat general belief that 
these distant countries must at one time or other have been 
united ; a belief which, after a careful examination of all the 
facts, does not seem to the author of this work to be well 
founded. On the other hand, there is the most satisfactory 
evidence that each southern region has been more or less 
closely united (during the tertiary or later secondary epoch) 
with the great northern continents, leading to numerous resem- 
blances and affinities in their productions. 
In endeavouring to present at a glance in the most convenient 
manner, the distribution of the families in the several regions 
and sub-regions, it was necessary to arrange them, so that those 
whose relations to each other were closest should stand side by 
side; the first and last being those between which the relations 
were least numerous and least important. Influenced by the 
usual opinions as to the relations between Australia and South 
America, the series was at first begun with the Nearctic, and 
terminated with the Australian and Neotropical regions ; and it 
was not till the whole of the vertebrate families had been gone 
through, and their distribution carefully studied, that these last 
two regions were seen to be really wider apart than any others 
of the series. It was therefore decided to alter the arrangement, 
beginning with the Neotropical, and ending with the Australian 
