1054 PRINXIPLES ()!• STRATIGRArHV 



arable from one another and wliicli \aried from period to period. 

 Even within the same period the geographic provinces are not 

 constant for all animals or plants, because some groups of animals 

 and plants are not affected by barriers which are restrictive to 

 others. Nevertheless a general division of the earth's surface into 

 biotic provinces is possible, up to a certain extent ; at least within 

 the marine and terrestrial realms. Within these two realms the 

 distribution of organisms is the resultant of migration and dis- 

 persal, not only of the organisms themselves, but also of their 

 ancestors in preceding geologic periods. Thus the problem of 

 geographic distribution of a given group of organisms resolves itself 

 into a study of the migrations and dispersions of the ancestral types 

 of these organisms from the point of their origin in past time. 

 From the point of origin or center of distribution a process of 

 adaptive radiation has carried the organism outward and onward in 

 space and time. Barriers arising across the path of distribution break 

 the continuity of the range ; and from these separated remnants, or 

 relicts, arise new diversified types, along a number of distinct lines 

 radiating from the central stock, the resultants being in general 

 well adapted each to a particular phase of the environment. In 

 other distinct provinces similar lines of evolution may give rise 

 to parallel series of modifications ; so that, in provinces widely sep- 

 arated, similar lines may arise independently, the closeness of the 

 resemblance seemingly indicating a close geographic connection 

 between the provinces. The species of the gastropod genus Clavi- 

 lithes are an excellent example of this phenomenon. This genus 

 is represented by a number of very marked species in the Eocenic 

 strata of the Paris Basin. A parallel series occurs in the London- 

 Hampshire Eocenic basin, but there are scarcely any identical spe- 

 cies. Another parallel series occurs in the Gulf Eocenic of the 

 United States, a province entirely distinct from, and without com- 

 munication with, the European provinces, which themselves were 

 closely circumscribed. Nevertheless, the American series has the 

 same specific types as the Paris series, corresponding species of 

 the two generic stocks being scarcely distinguishable, so far as the 

 specific characters are concerned ; though it is an easy matter to 

 separate them generically — that is. to place each species in its 

 proper genetic series. If, as seems highly probable, these two 

 groups of Clavilithids had a common ancestor, in early Eocenic, 

 Palseocenic, or late Cretacic time, they became entirely separated 

 in mid-Eocenic time, and the parallel specific types arose indepen- 

 dently. 



