AZGENTURY OF PROGRESS: IN PALEONTOLOGY 507 
German, which included elements from each. In the same way 
a resultant fauna, formed by the mingling of two or more faunas, 
will contain elements from each which can be detected by a care- 
ful study and dissection of the entire assemblage of organisms. 
In general, the species in any fossil fauna fall into one of two 
categories: (1) indigenous evolution species, or those whose 
ancestors have lived in the same region and whose evolution has 
there taken place, and (2) immigration species whose ancestors 
have lived in some other geologic province and whose sudden 
appearance in the fauna under consideration is due to an immi- 
gration from the outside. It is the work of the paleontologic 
geologist to investigate each species of the fauna he may be 
studying, to determine in which category each belongs, and in 
the case of the immigration species to determine in what part of 
the world or in what geologic province its previous evolution 
has taken place, and what are the probable paths of migration. 
In the historical study of fossil faunas it is interesting and 
suggestive to draw parallels with the history of human races. 
Until comparatively recent times there has been a remarkable 
provincial development of the races of men. Among the more 
primitive peoples a range of mountains or a great river was a 
sufficient barrier to prevent migration, because, as in the case of 
the provincial faunas, they had not means at hand for intercom- 
munication between the separated provinces. But there has 
been a gradual development among the human races, of means of 
communication, first by beasts of burden and later by mechanical 
devices, until today, through the agency of the great ocean 
steamers, the transcontinental railways, and the telegraph, no 
corner of the inhabitable earth can be said to be isolated. With 
the increased facilities for intercommunication, there is being 
developed a great cosmopolitan human race, which will in time 
inhabit the whole earth, just as in far-away Silurian time there 
was developed, by reason of the widespread shallow waters upon 
the continents, a cosmopolitan fauna which reached from North 
America to Australia. 
The questions involved in this faunal history of the earth are 
