40 PRINCIPLES OF PALAEONTOLOGY. 



namely, a generalisation has been established that certain 

 fossils occur in strata of a certain age, palaeontologists are apt 

 to infer that all beds containing similar fossils must be of the 

 same age. There is a presumption, of course, that this infer- 

 ence would be correct; but it is not a conclusion resting upon 

 absolute necessity, and there might be physical evidence to 

 disprove it. Fifthly, the physical geologist may lead the palae- 

 ontologist astray by asserting that the physical evidence as to 

 the age and position of a given group of beds is clear and un- 

 equivocal, when such evidence may be, in reality, very slight 

 and doubtful. In this way, the observer may be readily led 

 into wrong conclusions as to the nature of the organic remains 

 often obscure and fragmentary which it is his business to 

 examine, or he may be led erroneously to think that previous 

 generalisations as to the age of certain kinds of fossils are 

 premature and incorrect. Lastly, there are cases in which, 

 owing to the limited exposure of the beds, to their being 

 merely of local development, or to other causes, the physical 

 evidence as to the age of a given group of strata may be en- 

 tirely uncertain and unreliable, and in which, therefore, the 

 observer has to rely wholly upon the fossils which he may 

 meet with. 



In spite of the above limitations and fallacies, there can be 

 no doubt as to the enormous value of palaeontology in enab- 

 ling us to work out the historical succession of the sedimentary 

 rocks. It may even be said that in any case where there 

 should appear to be a clear and decisive discordance between 

 the physical and the palseontological evidence as to the age 

 of a given series of beds, it is the former that is to be distrusted 

 rather than the latter. The records of geological science con- 

 tain not a few cases in which apparently clear physical evi- 

 dence of superposition has been demonstrated to have been 

 wrongly interpreted ; but the evidence of palaeontology, when 

 in any way sufficient, has rarely been upset by subsequent 

 investigations. Should we find strata containing plants of the 

 Coal-measures apparently resting upon other strata with Am- 

 monites and Belemnites, we may be sure that the physical 

 evidence is delusive ; and though the above is an extreme case, 

 the presumption in all such instances is rather that the physical 

 succession has been misunderstood or misconstrued, than that 

 there has been a subversion of the recognised succession of 

 life-forms. 



We have seen, then, that as the collective result of observa- 

 tions made upon the superposition of rocks in different locali- 

 ties, from their mineral characters, and from their included 



