332 Geographical Distribution of Animals 



Chatham Island. Morphologically they may well form but one genus, 

 since they have sprung from the same stock and have developed upon 

 the same lines ; they are therefore monogenetic : but since we know 

 that they have become what they are independently of each other 

 (now unlike any other Rails), they are polygenetic and therefore 

 could not form one genus in the old Darwinian sense. Further, they 

 are not a case of convergence, since their ancestry is not divergent 

 but leads into the same stratum. 



Tlte reconstruction of the geography of successive epochs. 



A promising method is the study by the specialist of a large, widely 

 distributed group of animals from an evolutionary point of view. Good 

 examples of this method are afforded by A. E. Ortmann's 1 exhaus- 

 tive paper and by A. W. Grabau's "Phylogeny of Fusus and its 

 Allies " (Smithsonian Misc. Coll. 44, 1904). After many important 

 groups of animals have been treated in this way as yet sparingly 

 attempted the results as to hypothetical land-connections etc. are 

 sure to be corrective and supplementary, and their problems will be 

 solved, since they are not imaginary. 



The same problems are attacked, in the reverse way, by starting 

 with the whole fauna of a country and thence, so to speak, letting 

 the research radiate. Some groups will be considered as autoch- 

 thonous, others as immigrants, and the directions followed by them 

 will be inquired into ; the search may lead far and in various direc- 

 tions, and by comparison of results, by making compound maps, certain 

 routes will assume definite shape, and if they lead across straits and 

 seas they are warrants to search for land-connections in the past 2 . 

 There are now not a few maps purporting to show the outlines of 

 land and water at various epochs. Many of these attempts do not 

 tally with each other, owing to the lamentable deficiencies of geological 

 and fossil data, but the bolder the hypothetical outlines are drawn, 

 the better, and this is preferable to the insertion of bays and similar 

 detail which give such maps a fallacious look of certainty where none 

 exists. Moreover it must be borne in mind that, when we draw a 

 broad continental belt across an ocean, this belt need never have 

 existed in its entirety at any one time. The features of dispersal, 

 intended to be explained by it, would be accomplished just as well 

 by an unknown number of islands which have joined into larger com- 

 plexes while elsewhere they subsided again : like pontoon-bridges 



1 " The geographical distribution of Freshwater Decapods and its bearing upon ancient 

 geography," Proc. Amer. Phil. Soc. Vol. 41, 1902. 



2 A fair sample of this method is C. H. Eigenmanu's "The Freshwater Fishes of 

 South and Middle America," Popular Science Monthly, Vol. G8, 1906. 



