8Q INTRODUCTION. 



fossil animals, indeed, possess peculiarities so great as to 

 entitle them to be placed in any class, other than in one of 

 the classes of recent forms. On the other hand, the differ- 

 ences between some of the ancient types of life and the 

 existing ones are so great, that palaeontologists have been 

 compelled to construct new sub-classes, orders, and genera for 

 their reception. Moreover, many fossil animals are not only 

 very different from living ones, but they are often " compre- 

 hensive " or " transitional " in their characters. In other 

 words, fossil animals are often " comprehensive types," and 

 combine in themselves characters now only found separ- 

 ate, thus serving as " transitional links " between groups 

 which are at present widely removed from one another. 

 For example, the reptiles and the birds form at the present 

 day two widely separated classes, but some fossil birds 

 exhibit peculiarities of a distinctly reptilian character, and 

 some fossil reptiles approximate to the birds in structure ; 

 and the fossil forms thus partially fill up the gap w T hich now 

 exists between these two great divisions of the animal 

 kingdom. 



Again, many fossil animals exhibit what are termed 

 " generalised " characters. If, namely, we construct for our- 

 selves a " general " or " ideal " type for any great group of 

 animals a type which shall possess all the essential char- 

 acters of the group, without its non-essential ones then we 

 find that the fossil animals of the same group are generally 

 nearer to this type than are its living representatives. 

 Moreover, the older representatives of any given group are 

 usually nearer to the ideal type of the group or are more 

 "generalised" than are the later representatives of the 

 same group. All zoologists, however, admit that the process 

 of development in any individual animal is one in which 

 there is a gradual progress from the general to the special, 

 the embryo being nearer to the general type of the group 

 to which it belongs than the adult is. In other words, the 

 embryo animal is more generalised than the adult, and the 

 process of development is one of specialisation. Admitting 

 this, it follows that the fossil forms belonging to any given 

 group, in so far as they are " generalised " in their characters, 



