SOME ASPECTS OF GEOLOGIC TIME 273 



the changes in the earth structures of early times. The point I 

 am specially concerned to urge is this : If, in any group of strata, 

 one species suddenly vanishes, and another allied species 

 suddenly takes its place, which is exactly what does appear to 

 occur, there is prima facie evidence for a considerable gap in the 

 succession of the rocks, 



The remarkable succession of " Oppel's zones " gives rise to 

 many interesting questions. The more detailed information we 

 can get the better. We require, from the researches of specialist 

 geologists, a clear answer to a series of questions such as the 

 following : 



(a) Is the species marking what we will call a zone identical 

 at its base and at its summit ? 



(b) Is the species identical at the base in India and at the base 

 in Europe, at the base in India and the summit in Europe ? If 

 not, what, so far as can be discovered, is the extent of the varia- 

 tion for time and space ? 



(c) In each district containing a fairly complete series of 

 Jurassic beds, what zones are present and what are absent ? 



And so on. 



Facts such as these are probably known. The zonal classi- 

 fication probably merely implies that certain dominant forms 

 occur in a definite order. When such a classification is made 

 the essential point occurs in locating a gap. The fact that 

 certain zones are missing in certain groups of strata in certain 

 districts has a clear and definite meaning. But the point of 

 greatest interest is found in the gaps, and particularly in gaps 

 that appear to be world-wide. Here we come somewhere near 

 bedrock in our co-ordination of organic evolution and geologic 

 time. If in certain strata we find a sudden disappearance of 

 form (a), and a sudden replacement for it of form (£), and 

 we find no strata in which form (a) and form (b) are found 

 together, the natural inference is that a considerable interval 

 of time has elapsed between the two depositions. If any- 

 where in strata roughly contemporaneous we can discover a 

 filling of the evolutionary gap, either the two forms occurring 

 together or the existence of forms intermediate between the 

 two, the problem of the intermission is partially solved. If 

 nowhere in any strata are intermediate forms to be found, and 

 if, as appears to be the case, fossils (a) and (b) are plentiful in 

 their respective zones, are never found together, and inter- 

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