The Peinciples of Palaeontology. 149 



But a new objection presents itself which brings us back 

 directly to our subject. In the examination of successive geologic 

 beds, we ought always to be able to find transition types 

 between two distinct species, either under the form of local 

 varieties, or else under the form of mutations. But, on the con- 

 trary (as it is claimed). Palaeontology fails to furnish any such 

 indications. 



Darwin replied to this argument by laying stress on the insuf- 

 ficiency of geologic documents, an insufficiency concerning both 

 the difficulties of fossilization and the relative scarcity of 

 materials acquired. Since that period, attention having been 

 directed toward this class of studies, the transition types 

 discovered have become more and more numerous. We have 

 cited some of these, and in the course of the following chapters 

 we shall, little by little, give descriptions of them. Still it 

 remains an indisputable fact that in the most thoroughly explored 

 regions, those where the fauna is best known, as, for instance, the 

 Tertiary of the Paris basin, the species of one bed often differ" 

 widely from those of the preceding, even where no stratigraphic 

 gap appears between them. This is easily explained. The pro- 

 duction of new forms usually takes place within narrowly 

 limited regions. It may happen in reality that one form evolves 

 in the same manner in localities widely separated from each 

 other, and farther on we shall see examples of this; but this is 

 not generally the case, the area of the appearance of species is 

 usually very circumscribed. This fact has been established in the 

 case of certain existing butterflies and plants.* The diversity 

 having once occurred, the new types spread often to great dis- 

 tances, and may be found near the present form without crossing 

 with it or presenting any trace of transition. 



The same phenomenon must have taken place in former epochs. 

 It is then only by the merest chance that geologists are able to 

 locate the origin of the species they have under consideration; 

 if, furthermore, the phenomena of erosion or metamorphism 

 have destroyed or changed the locality in question, direct obser- 

 vation will not furnish any means of supplying the missing links 

 of the chain. Nevertheless in certain places rich in fossils, where 

 the superposition of the deposits is without interruption, sorne 



* Bates, The Xaturalist on the Amazons. London, 1863. 



