40 PRINCIPLES OF PALAZZSONTOLOGY. 
namely, a generalisation has been established that certain 
fossils occur in strata of a certain age, paleeontologists are apt 
to infer that a 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 palez- 
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 palzontology 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 paleontological 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 palzeontology, 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 
