PKEFACE. 



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 luust 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. I 



