PREFACE 
ix 
by itself; and it seems better to omit it altogether from a 
zoological work, than to treat it in a necessarily superficial 
manner. 
The best method of illustrating a work of this kind was a 
matter requiring much consideration. To have had a separate 
coloured or shaded map for each family would have made 
the work too costly, as the terrestrial vertebrates alone 
would have required more than three hundred maps. I had 
also doubts about the value of this mode of illustration, as it 
seemed rather to attract attention to details than to favour the 
development of general views. I determined therefore to adopt 
a plan, suggested in conversation by Professor Newton ; and to 
have one general map, showing the regions and sub-regions, 
which could be referred to by means of a series of numbers. 
These references I give in the form of diagrammatic headings 
to each family ; and, when the map has become familiar, 
these will, I believe, convey at a glance a body of important 
information. 
Taking advantage of the recent extension of our knowledge 
of the depths of the great oceans, I determined to give upon this 
map a summary of our knowledge of the contours of the ocean 
bed, by means of tints of colour increasing in intensity with 
the depth. Such a map, when it can be made generally accurate, 
will be of the greatest service in forming an estimate of the 
more probable changes of sea and land during the Tertiary 
period ; and it is on the effects of such changes that any satis- 
factory explanation of the facts of distribution must to a great 
extent depend. 
Other important factors in determining the actual distribution 
of animals are, the zones of altitude above the sea level, and the 
strongly contrasted character of the surface as regards vege- 
tation — a primary condition for the support of animal life. X 
