CRITICAL PERIODS IN THE HISTORY OF THE EARTH. 529 



Or perhaps, more acearately, we might say that the existing European 

 mammalian fauna is a resultant of all these factors, but the controlling fac- 

 tor is the Asiatic. With the Asiatic invasion came man. and was a prime 

 agent in determining the final result. 



Thus, regarding the Tertiary and the Present as consecutive eras, and 

 the Quaternary as the transition or critical period between, then, if the 

 record of this period had been lost, corresponding with the unconformity here 

 found, we should have had here an enormous and apparently sudden change 

 of mammalian species. Yet this change of fauna, as great as it is, is not tO' 

 be compared with that which occurred between the Archaean and Palfeozoic,. 

 or between the Palaeozoic and Mesozoic, or even that between the Mesozoie 

 and Csenozoic; for the change during the Quaternary is mostly confined tO' 

 species of the higher mam.mals, while the change during previous critical 

 periods extended to species of ail grades, and not only to sjaecies, but to gen- 

 era, families, and even orders. We conclude, therefore, that the previous 

 critical periods or lost interrals were far longer than the whole Quaternary : 

 or else that the rate of evolution was far more rapid in these earlier times. 



To sum uj), then, in a few words, the general formal laws of evolution- 

 change throughout the whole history of the earth : — 



(1.) Gradual, very slow changes of form everywhere under the influence 

 of all the factors of change, known and unknown : for example, pressure 

 of changing physical conditions whether modifying the individual (cer- 

 tainly one fiactor), or selecting the fittest offspring (certainly another fac- 

 tor); improvement of organs by use and the improvement inherited (cer- 

 tainly a third factor), and perhaps still other factors yet unknown. This 

 general evolution by itself considered would produce similar changes every- 

 where, and therefore would produce geological faun^. but not geographical 

 diversity. Determination of a geological horizon would in this case be easy, 

 because fossil species would be everywhere identical. 



(2.) Changes in different places and under different physical conditions, 

 taking different directions and advancing at different rates, give rise to 

 geographical faunae. This, if there were nothing more, would produce far 

 greater geographical diversity and more complete localization of faunjeand 

 florae than now exist, — so great that the determination of a geological hori- 

 zon would be impossible. 



(3.) The force of.^change resisted by heredity, in some species and gen- 

 era more than in others, determines paroxysms of more rapid movement 

 of general evolution, affecting sometimes species, sometimes genera or fam- 

 ilies. The sudden appearance of species, genera, families, etc., in quiet 

 times is thus accounted for, 



(4.) During critical periods, oscillations of the crust, with rapid chano-es of 

 physical geography and climate, determine a more rapid rate of change in 

 all forms; first, by greater pressure of physical conditions; and, second, by 

 migrations partly enforced by the changes of climate and partly permitted 

 by removal of barriers, and the consequent invasion ot one fauna and flora 



