﻿Ixvi 



PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [vol. lxxii, 



efficiency o£ the brain, which culminated in Man. If, in these 

 circumstances, the same type of animal has originated more than 

 once, it should he possible, in some cases at least, to discover 

 the phenomenon and determine its limitations. 



Although the enquiries involved are almost entirely biological, 

 it is especially important for the geologist to take note of them, 

 because students of shells from the modern standpoint are 

 unanimous in recognizing what they term homceomorph} r . 

 They continually find ammonites, gastropods, and brachiopods, for 

 example, which are essentially identical when full-grown but differ 

 completely in their early stages. When the fossils happen to be 

 perfect, these early stages are, of course, preserved and distin- 

 guishable ; and if they reproduce, even only approximately, the 

 ancestral condition in each case, the}' certainly suggest that the 

 same type of shell may have arisen from more than one source. 

 If this be so, shells from different horizons and widely-separated 

 localities need very careful scrutiny before they can be used for 

 correlating geological formations ; for, even if nearly similar, they 

 may represent animals that have no really close affinity, and any 

 diagnostic features that they show may be inconspicuous points 

 which have generally been overlooked as insignificant. For a 

 clearer view of general principles, we therefore turn with ex- 

 pectancy to vertebrate skeletons, which have much more numerous 

 and tangible characters, and approach senility in more varied ways. 



Even among vertebrates it is by no means easy in every case to 

 interpret the evidence that most concerns the geologist, and I 

 need only refer to the supposed thylacines (Sparassodonts) and 

 the horned tortoises (Miolania) of the Argentine Tertiary, which 

 have often been quoted as specially strong proofs of the former 

 existence of an Antarctic continent uniting the Australian and 

 South American regions. So far as can be determined from 

 their fossil remains, the Sparassodonts of the Santa Cruz Beds 

 of Patagonia differ only in • the slightest particulars from the 

 Pleistocene and existing thylacines of Australia, and they have 

 been placed in the same family by American palaeontologists. It 

 is, however, to be noted that the thylacines and Sparassodonts 

 are essentially identical with the primitive Creodonta, which are 

 known to have ranged over all the continental land of the 

 Northern Hemisphere at the beginning of the Tertiary Era. 

 They only differ from these early mammals in certain senile 

 specializations which might be expected in any long-lived group ; 



