104 INTRODUCTION. 



ral progression of zoological types, indicated by the successive intro- 

 duction of the great classes of Vertebrates in the order of their 

 zoological rank, is amply confirmed and strengthened by what is 

 known as to the geological history of all the great groups of animals 

 of which we have a fairly complete record. Even as regards the 

 Invertebrate sub-kingdoms, where we are admittedly ignorant of the 

 first appearance of the main divisions, we are nevertheless commonly 

 able to show that the minor groups have been successively intro- 

 duced in the order of their zoological rank. At the same time, it is 

 to be borne in mind that zoological groups do not usually appear for 

 the first time in either their lowest or highest forms. The earliest 

 representatives of each group are, on the other hand, very usually 

 " generalised " types, which have the capacity for either elevation or 

 degeneration in their later development. 



9. The Absence of closely graduated transitional Forms between 

 allied Morphological Types. — While the general testimony of Palaeon- 

 tology is overwhelmingly in favour of the view that some general law 

 of evolution has operated in the production of the varied forms of 

 life which now exist or have existed in the past, there is no direct 

 palaeontological evidence which would certainly establish any partic- 

 ular theory as to the precise modus operandi of this law. With 

 regard more particularly to the theory of " the origin of species by 

 means of natural selection," which the world owes to the genius of 

 Darwin, the evidence of Palaeontology cannot be said to be conclusive. 

 More especially, we have in most cases no sufficient evidence of the 

 former existence of the numerous and closely graduated transitional 

 forms between different species, which must, on this theory, have once 

 existed. It is an essential part of the theory of natural selection 

 that the production of any given species from any pre-existing species 

 can only have been effected through the intervention of a long series 

 of intermediate or transitional forms. It is true that many extinct 

 animals are known which are clearly transitional between existing 

 groups, now more or less widely separated from one another. It is 

 also true that in a certain number of instances (particularly among the 

 Molluscs) it has been found possible to connect two different specific 

 types by means of a long series of intermediate links, the separate 

 members of the series differing only in minute characters. As a 

 general rule, however, the known transitional forms between allied 

 groups are few in number, and are so far isolated from one another 

 and from the forms they connect as to be in themselves absolutely 

 distinct morphological types. It cannot be denied, therefore, that 

 palaeontology has, so far, to a large extent, failed to bring forward 

 the numerous and closely graduated series of intermediate forms 

 which must at one time have existed, supposing " natural selection " 

 to be the sole agent in the origination of new species. The absence 



