STRATIGRAPHICAL PALAEONTOLOGY '-'1 



and it is (luring these very periods of subsidence that the deposit* 

 which are richest in fossils have been accumulated." 1 



Succession of Faunas. From the consideration* above 

 mentioned it will lie seen that we cannot expect to find all th- 

 transitional forms which, if the theory of evolution be correct, 

 must have connected one species with another, and have linked 

 together all the past and present species of any one genus into 

 one long chain of life. All that we can hope to find are certain 

 cases where so large a number of closely allied species and varieties 

 have been preserved as to furnish us with strong evidence of their 

 having been links in such a chain. 



There are, moreover, two well-established generalisations from 

 palaeontological facts which lead to a very significant inference ; 

 these considerations are (1) that the assemblage of fossils found in 

 any formation has a general character intermediate between the 

 assemblages above and below : and (2) that the fossils from two 

 consecutive formations are far more closely related to each other 

 than the fossils from two more remote formations. Commenting 

 on these Darwin observes : 2 " On the theory of descent, the full 

 meaning of the fossil remains from closely consecutive formations 

 being closely related, though ranked as distinct species, is obvious. 

 As the accumulation of each formation has often been interrupted, 

 we ought not to expect to find in any one, or in any two, formations 

 all the intermediate varieties between the species which appeared 

 at the commencement and close of these periods ; but we ought 

 to find, after intervals very long as measured by years, but only 

 moderately long as measured geologically, closely allied forms, 

 or, as they have been called by some authors, representative 

 species ; and these assuredly we do find. We find, in short, such 

 evidence of the slow and scarcely sensible mutations of specific 

 forms as we have the right to expect" 



In spite, therefore, of the imperfection of the record, the known 

 succession of life-forms does give very great support to the theory 

 of evolution. Every succeeding fossil assemblage contains not 

 only a larger number and greater variety of forms, but also some 

 which are distinctly more specialised and more advanced in 

 organisation. It is an acknowledged fact that many of the more 

 ancient forms of life were what are termed " comprehensive types," 

 i.e. types which combine the structural characters of two or more 

 different orders or classes of creatures, which are now com] 

 differentiated from each other. In many cases also ancient animals 

 resemble the embryos of the more recent animals belonging to the 

 same classes. 



1 Op. cit. p. '27.'.. 2 Op. fit. sixth edition, p. 307. 



