SOME GENERAL OBSERVATIONS ON THE WHOLE QUATERNARY. 597 



This very important subject has been more closely studied in Europe 

 than here, although we believe that America is the simplest and best 

 field for its elucidation. During the Quaternary probably at least four 

 distinct mammalian faunas struggled together for mastery on European 

 soil : 1. The Pliocene autochthones. 2. Invasions from Africa, per- 

 mitted by geographical connection opening a gateway through the 

 Mediterranean, since closed. 3. Invasions from Asia, by opening of a 

 gateway which has remained open ever since ; with this invasion prob- 

 ably came man. 4. Invasions from arctic regions. Probably more than 

 one such invasion took place ; certainly one occurred during the second 

 Glacial epoch, called on that account the Reindeer period. 



The final result of all this struggle was, that the Pliocene autoch- 

 thones were destroyed or driven southward in Africa; the southern 

 species were mostly destroyed or driven back with changed forms and 

 diminished size ; the northern species, reindeer, glutton, etc., retreated 

 again northward, and the Asiatics remained in possession of the field, 

 but greatly changed by the struggle. Man was among these, and cer- 

 tainly one of the principal agents in the change. Speaking more ac- 

 curately, the present fauna of Europe may be said to be a product of 

 all these factors ; but the Asiatic invasion seems to be the largest factor. 



Thus, then, the gradual progress of evolution through all geological 

 time, and the causes of the phenomenon of rapid change of species at 

 critical periods of the earth's history, may be briefly summarized as 

 follows : 



1. A gradual, extremely sloiv evolution of organic forms under the 

 operation of all the forces and factors of evolution known and un- 

 known, whatever we may conceive these to be. This cause acting 

 alone would produce gradual changes in time (geological faunas), but 

 without geographical diversity. 



2. This slow evolution takes different directions in different places 

 and under different physical conditions, and thus gives rise to geo- 

 graphical faunas and floras. Such geographical faunas and floras, 

 if isolated by physical barriers, become more and more diverse so long 

 as the barriers are maintained. This cause acting alone would produce 

 extreme geographical diversity, and render determination of sjmchro- 

 nism impossible. 



3. During critical periods physical changes and consequent migra- 

 tions, partly enforced by changes of climate, partly permitted by re- 

 moval of barriers, and the precipitation of adjacent faunas and floras 

 upon each other, and the consequent severe struggle for life, give rise 

 to far more rapid changes of species, but at the same time to greater 

 geographical uniformity. This more rapid change of organic forms 

 is produced partly by severer pressure of external conditions, certainly 

 one factor of change ; partly by severer struggle for life, certainly an- 



