400 RETROSPECT OF THE OOLITIC SYSTEM. CHAP. 



Wenlock groups, and then 'dies out' in the uppermost Ludlow 

 rocks. The Devonian and carboniferous groups give another 

 example of the same order ; in fact, every great natural system of 

 strata which includes littoral, shallow, and deep sea deposits, offers 

 similar results in each limited natural district a beginning, a pro- 

 gress, an end, of many though not all the forms. 



The result here presented is derived from studying limited 

 natural districts ; when many separate districts are combined in 

 a general view, the conclusion is somewhat modified. Thus the 

 poor English series of poikilitic rocks loses in a general argument 

 its character of sterility and separation, and acquires many liassic 

 affinities in the muschelkalk, and some Permian alliances in the 

 lower sandstones. By this means the 'beginning' of the great 

 liassic and oolitic fauna is carried in Europe to a somewhat 

 earlier point of geological time than in England ; when we reach 

 that point its main features end in a reduced though considerable 

 number of forms for which no ancestors can be found, mixed 

 with others which seem to claim an earlier pedigree. Out of 

 observations of this kind, on the one hand, indicating original 

 diversities the idea has arisen of several successive and separate 

 systems of living beings adapted to successive conditions of the 

 globe ; on the other, the recurring or continued agreements seemed 

 to be best explained by 'descent with modification' from a common 

 ancestry. These general ideas can never be absent from the mind 

 of the geological student, and have indeed become subjects of 

 popular discussion, often with a very inadequate estimation of 

 the phenomena to be explained. 



In a limited sea-basin, in one system of strata, deposited in 

 similar conditions, with continuous life, the distinctive forms of 

 the several genera ranged in order of time furnish evidence in 

 the most complete form we are likely ever to obtain it, whereby 

 the hypotheses referred to can be examined, or rather one of them, 

 that of ' descent with modification.' In the Oxford district of 

 oolites we have such conditions, and may select as many examples 

 as we please of 'continued forms' of life. Choosing in preference 

 sedentary rather than erratic races, we may limit ourselves to a 

 few examples of brachiopoda, monomyaria, and dimyaria, and 

 place well-known species of each in the order of time all the 

 genera being still existent. 



