REASONING IN A CIRCLE. 33Q 



&quot;But,&quot; it will be replied, &quot;in past eras the same, or 

 similar, organic forms were more widely distributed than 

 now.&quot; It may be so ; but the evidence adduced by no 

 means proves it. The argument by which this conclusion 

 is reached, runs a risk of being quoted as an example of 

 reasoning in a circle. As already pointed out, between 

 formations in remote regions there is no means of ascertain 

 ing equivalence but by fossils. If, then, the contempora 

 neity of remote formations is concluded from the likeness 

 of their fossils ; how can it be said that similar plants and 

 animals were once more widely distributed, because they 

 are found in contemporaneous strata in remote regions ? 

 Is not the fallacy manifest ? Even supposing there were 

 no such fatal objection as this, the evidence commonly as 

 signed would still be insufficient. For we must bear in 

 mind that the community of organic remains commonly 

 thought sufficient for inferring correspondence in time, is a 

 very imperfect community. When the compared sedimen 

 tary beds are far apart, it is scarcely expected that there 

 will be many species common to the two : it is enough if 

 there be discovered a considerable number of common gen 

 era. Now had it been proved that, throughout geologic 

 time, each genus lived but for a short period a period 

 measured by a single group of strata something might be 

 inferred. But what if we learn that many of the same 

 genera continued to exist throughout enormous epochs, 

 measured by several vast systems of strata ? &quot; Among 

 molluscs, the genera Avicula, Mbdiola, Tcrebratula, Lin- 

 gnla, and Orbicida, arc found from the Silurian rocks up 

 wards to the present day.&quot; If, then, between the lowest 

 fossiliferous formations and the most recent, there exists 

 this degree of community ; must we not infer that there 

 will probably often exist a degree of community between 

 trata that arc far from contemporaneous ? 



Thus the reasoning from which it is concluded that 



