FEBEtlART 18, 1910] 



SCIENCE 



253 



In the circling changes of geography 

 such an epoch of divei-sity has been fol- 

 lowed by the development of more or less 

 extensive uniformity, according to the 

 periodicity of diastrophism. Let it be as- 

 sumed that in the course of a long period 

 of quiet, barriers yielded to the monotony 

 of low lands, freely communicating seas, 

 and genial cosmopolitan climates. The 

 factors of evolution were then profoundly 

 generalized. Isolation was replaced by 

 intercourse, adaptation by competitive de- 

 velopment and variation, restriction by op- 

 portunity. Success lay with him who had 

 the intrinsic capacity to occupy and to 

 hold the widening realm of life. Out of 

 such conditions came cosmopolitan faunas, 

 which exhibit closely similar or identical 

 associations of species even though inhab- 

 iting widely separated regions. The 

 identity may be due to perpetuation of 

 ancestral species, which have followed up 

 the movement of a favoring habitat; or it 

 may result from evolution of a successful 

 fauna, competent to spread throughout the 

 wide kingdom to which it is born. In the 

 one case the migrants may have lived sim- 

 ultaneously with descendants of the com- 

 mon ancestors in the home province, or the 

 ancestral stock may have died out there be- 

 fore the migration Avas complete. T/ie 

 iime equivalent or coefficient of migration 

 is indeterminate. In the second case, that 

 of indigenous evolution, the time elapsed 

 while the species spread over an area which 

 was everywhere geographically favorable 

 depended only upon the ability of the mi- 

 grant and may be assumed to have been 

 brief as compared with geologic epochs. 

 This is the usual assumption. It may be 

 true for appropriate species and periods, 

 but is by no means always true. 



Cosmopolitan conditions have been truly 

 world-wide only in exceptional cases. 

 Very extended faunal provinces have been 



less rarely developed. The Arctic Ocean 

 has been one which repeatedly expanded 

 to include much of Eurasia and America. 

 The girdle of ocean currents which en- 

 circled the world in the northern, tem- 

 perate and tropical zones during Paleozoic, 

 Mesozoic and Eocene times was another 

 such province. Both of these became from 

 time to time the homes of cosmopolitan 

 faunas that existed simultaneously over 

 surprisingly wide realms. At other times 

 they were restricted or divided and faunas 

 became provincial. 



If we consider the course of evolution 

 during an epoch when general conditions 

 yielded to provincial environments (ex- 

 cluding the case in which the change is too 

 drastic) the law which applies is Jordan's 

 law of isolation. He uses that term to 

 signify the separation of one or many in- 

 dividuals from others of their kind. The 

 separation implies more or less diversity 

 of environment and consequently more or 

 less unequal or unlike variation along the 

 many possible paths open to the living or- 

 ganism. "We conceive a broad life realm, 

 marine or terrestrial, which through subtle 

 changes in the flow of currents of the sea, 

 or of climates of the land, or of depth of 

 waters, or of altitudes above the seas, or of 

 any other condition affecting sensitive or- 

 ganisms, is divided into provinces which 

 offer unlike environments to the descend- 

 ants of the ancestral cosmopolitan fauna. 

 Adaptation becomes again the dominant 

 process. Being variously conditioned, it 

 leads to variation and the development of 

 different species. 



North America represents the facts upon 

 which Jordan" founded the law of twin 

 species, which is that : 



"Jordan, David Starr, Isolation as a Factor in 

 Organic Evolution, in " Fifty Years of Darwin- 

 ism," 1909, p. 73. 



