626 Original Articles. [Oct., 



happens that they are only analogous species. In illustration of 

 this latter point I may mention that in the Tertiary strata of 

 Victoria and South Australia are found a number of species of 

 Valuta with blunt spires,* but otherwise scarcely distinguishable 

 from the Eocene sharp-spired species of Voluta occurring in Britain 

 (compare Figs. 4 and 5 of the annexed Plate). But does it there- 

 fore follow that these two sets of strata, so widely distant from 

 one another, are of the same age ? By no means, as it seems to me. 

 For, leaving the distance out of the question, for fear of being charged 

 with the fallacy of "begging the question," all the actual evidence 

 (as distinguished from the ideal) goes to prove that the Australian 

 beds are much the more recent. In these beds the curious species 

 of Trigonia, which formed the subject of a note by me in the April 

 number of this Journal, has been found. It is represented in Fig. 6 

 of the Plate, and the type of the group to which it belongs (T. costata, 

 an Oolitic species) in Fig. 7. 



Zoologically, therefore, if two series of strata can be correlated 

 only by means of representative species, their actual ages ought to 

 differ more widely than those which have some species in common. 

 And observed facts really give strength to this inference, for it rarely 

 happens but that the less distant strata, presumably contemporaneous, 

 have more species in common than those which are more widely 

 separated. This argument may, of course, be pushed still farther, as 

 when the species cease to be evidently representatives, or when they 

 belong to different genera ; but it must always be borne in mind, that 

 it has no force whatever unless strong collateral evidence exists, such 

 as would ordinarily induce a geologist or a paleontologist to correlate 

 the one formation with the other. There are no strong lines in 

 nature, and when the affinity of the fossils of two formations becomes 

 very weak, it ordinarily happens that we must look above or below 

 the one selected as a standard for comparison, in order to find the 

 deposit which seems, prima facie, to be the nearest in age to the one 

 we wish to correlate. 



The fearful palaeontologist may ask what limits are to be placed 

 upon the apparent difference in age of two formations apparently con- 

 temporaneous. In mathematical language, what is the function with 

 which we are to differentiate the ages of apparently contemporaneous 

 strata ? It appears to me that we have that function in the term 

 Geographical Space ; for I believe that as the distance between the 

 formations becomes greater, so does, cceteris paribus, the difference 

 in age increase. In other words, Geographical Space and Geological 

 Time have the same sort of correlation as Electricity and Magnetism ; 

 with apparently contemporaneous strata a certain distance means a 

 certain lapse of time, and as one varies so does the other. Still, this 

 is an individual opinion, and it does not quite coincide with that ex- 

 pressed by other writers. Professor Huxley, for example, states that, 

 ' : For anything that geology and palaeontology are able to show to the 

 contrary, a Devonian fauna and flora in the British Islands may have 



* Set Plot'. M'Coy's Contribution to the ' Victorian Essays,' p. 168. 



