THE EVOLUTION OF ORGANIC TYPES IN TIME. 1 01 



(a.) In the first place, it is a powerful argument in favour of the 

 theory of evolution that the primary morphological types which we 

 recognise among existing animals are identical with the types upon 

 which fossil animals are constructed. While the great majority of 

 fossils are extinct, and while many of them are extremely unlike any 

 existing forms, no fossil animal has hitherto been detected which 

 cannot be referred to one or other of the existing sub-kingdoms. Few 

 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 differences between some of the 

 ancient types of life and the existing ones are so great, that palaeon- 

 tologists have been compelled to construct new sub-classes, orders, 

 and genera for their reception. 



(b.) Again, the investigation of fossil animals has tended to greatly 

 diminish the intervals by which allied groups of existing animals are 

 separated. Many fossil animals, namely, are what has been termed 

 " comprehensive " in their morphological characters. That is to say, 

 they combine in themselves structural peculiarities which in later 

 formations or at the present day are only found separately, in groups 

 more or less widely removed from one another. These " synthetic 

 types " or " collective types " serve to bridge over the gaps between 

 related morphological types, and are thus of special interest from a 

 theoretical point of view. Thus, to take a single example only, the 

 two great classes of the Sauropsida — viz., the Reptiles and the 

 Birds — are at the present day separated by a wide interval. Palae- 

 ontology, however, has brought to light a number of transitional fos- 

 sil forms — some referable to Birds and some to Reptiles — which 

 more or less markedly combine in themselves the distinctive charac- 

 ters of both groups, and thus partially fill up the gap which now 

 exists between these two great divisions of Vertebrates. 



(c.) Again, many fossil animals exhibit what are termed "general- 

 ised " characters. If, namely, we construct for ourselves a " general " 

 or " ideal " type for any great group of animals — a type which shall 

 possess all the essential characters 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 represen- 

 tatives. Moreover, the older representatives of any given group are 

 usually nearer to the ideal type of the group — or are more "general- 

 ised" — 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, 



