Passage of Species. 173 



or about 14 per cent, pass into the Middle Lias ; and 

 of 500 species in the latter, 57, or about 11^ per cent, 

 pass into the Upper Lias ; while of 312 Upper Lias 

 species, 39, or about 1 2J per cent, pass into the Inferior 

 Oolite which succeeds it. 



Few biologists and geologists now believe in the 

 sudden extinction of entire old marine faunas, or 

 even of the greater part of them, and their equally 

 sudden replacement by new creations ; for it begins to 

 be generally understood that life is variable and pro- 

 gressive, the change of species in given areas being due 

 chiefly, in comparatively short epochs, to migrations 

 out of and into these areas, in consequence of changes 

 of local conditions, such as depth of water, and nature 

 of sediments, while in long periods of geological time, 

 it is best accounted for by that process of evolution so 

 clearly expounded by Darwin. Neither is it a fair 

 test of the community of species in two so-called form- 

 ations, to take the entire fauna of the lower one, and 

 calculate the percentage of forms that pass into the 

 overlying deposit, for, between the lower and upper 

 parts of many thick formations, there is often the same 

 kind of difference in assemblage of species that there 

 is between the adjoining parts of two so-called distinct 

 formations. In judging then of passage of species, if 

 we had all the data, the fairest method would generally 

 be to estimate the passage of forms by those in 

 common between the upper part of the lower formation 

 and the lower part of the upper one, in which case it 

 would often be found, when there is a natural confor- 

 mity between the strata, that the percentage of species 

 that pass onward is much increased. 



We now come to the Oolitic series of strata. 



On the flank of the Cots wold escarpment, south of 



