CHRONOLOGICAL SUCCESSION. 41 



begin with, many cases in which there is no palaeontological 

 evidence extant or available as to the age of a given group 

 of strata. In the second place, palaeontological observers in 

 different parts of the world are liable to give different names 

 to the same fossils, and in all parts of the world they are occa- 

 sionally liable to group together different fossils under the 

 same title. Both these sources of fallacy require to be guarded 

 against in reasoning as to the age of strata from their fossil 

 remains. Thirdly, the mere fact of fossils being found in beds 

 which are known by physical evidence to be of different ages, 

 has commonly led palaeontologists to describe them as dif- 

 ferent species. Thus the same fossil, occurring in successive 

 groups of strata, and with the merely trivial and varietal differ- 

 ences due to the gradual change in its environment, has been 

 repeatedly described as a distinct species, with a distinct name, 

 in every bed in which it was found. We know, however, that 

 many fossils range vertically through many groups of strata, and 

 there are some which even pass through several formations. 

 The mere fact of a difference of physical position ought never 

 to be taken into account at all in considering and determining 

 the true affinities of a fossil. Fourthly, the results of experience, 

 instead of being an assistance, are sometimes liable to operate 

 as a source of error. When once, namely, a generalization 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 inference 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 palaeontologist astray by asserting that the physical 

 evidence as to the age and position of a given group of beds is 

 clear and unequivocal, 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 

 generalizations 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 entirely uncertain 

 and unreliable, and in which, therefore, the observer has to rely 

 wholly upon the fossils which he may meet with. 



