44 INTRODUCTION. 



greater part of the North Temperate Zone, at a comparatively 

 recent geological period, has been visited with all the rigours 

 of an Arctic climate, resembling that of Greenland at the pre- 

 sent day. This is indicated by the occurrence of Arctic shells 

 in the superficial deposits of this period, whilst the Musk-ox 

 and the Reindeer roamed far south of their present limits. 



CHAPTER VI. 



DIVISIONS OF THE ANIMAL KINGDOM, AND 

 SUCCESSION OF ORGANIC TYPES. 



IT seems hardly necessary to remark that Palaeontology, as 

 a science, is based upon the kindred sciences of Zoology and 

 Botany, and that no satisfactory acquaintance with the former 

 can be arrived at without the previous acquisition of some 

 knowledge of the latter. It cannot be pretended to teach here 

 even the rudiments of these sciences, but there are a few points 

 which may be noticed as having a special bearing upon the 

 study of Palaeontology. 



CLASSIFICATION OF THE ANIMAL KINGDOM. Leaving the 

 vegetable kingdom till we come to speak of fossil plants, a 

 few remarks may be made on the classification of the animal 

 kingdom. Vast as is the number of known animals, all, 

 whether living or extinct, may be classed under some five or 

 six primary divisions or "morphological types," which are 

 technically spoken of as the " sub-kingdoms." All the animals 

 in any one sub-kingdom agree with one another' in their 

 structural type, or in the fundamental plan upon which they 

 are constructed ; and they differ from one another simply in 

 the modifications of this common plan. No comparison, 

 therefore, is possible between an animal belonging to one sub- 

 kingdom, and one belonging to another, since their distinguish- 

 ing characters are the result of the modification of two essen- 

 tially different ground-plans. Hence, it is possible to arrange 

 the animals of any one sub-kingdom in something like a linear 

 series, in which the lowest of the series most closely approaches 

 the primitive or ideal form of the sub-kingdom, whilst the 

 highest exhibits the greatest amount of complexity ancl special- 

 isation of this type. But it is not possible to establish any 

 such linear classification for the animal kingdom as a whole. 

 Given an animal of a lower "sub-kingdom" than another 



