Febbuaey 18, 1010] 



SCIENCE 



259 



of the geographic changes of the particular 

 period and of the geographic conditions 

 that preceded. 



The coefficient may he placed at a mini- 

 mum, which is possibly negligible, at times 

 of established cosmopolitan relations; but 

 it rises to a quantity which we can not neg- 

 lect at intervening periods of physical 

 change. 



The emphasis here placed on the geo- 

 graphic factor in correlation should not 

 obscure the initiative part played by the 

 life principle. It is the evolutionary force ; 

 its energy and the direction of its action 

 depend upon the kind of organism. But 

 the conditions of its action, its rate and the 

 result depend upon environment at any 

 instant and upon environmental change in 

 the long run. 



SUMMARY 



The broad general principles of paleo- 

 geography, which I would cite as most 

 fundamental, are as follows: 



1. Ocean basins are permanent hollows 

 of the earth's surface and have occupied 

 their present sites since an early date in 

 the development of geographic features. 

 This principle does not exclude notable 

 changes in the positions of their margins, 

 which on the whole have encroached upon 

 continental areas. 



2. Superficial oceanic circulation within 

 the permanent oceans has persisted since 

 an early stage of their existence, essentially 

 in the great drifts which it now follows 

 under the trade winds. It is probable that 

 the present deep circulation of oceanic 

 waters, poleward at the surface and 

 equatorward below the surface, is due to 

 exceptional refrigeration at the pole, and 

 has been preceded during past ages by a 

 prevailing reversed movement of warm 

 saline waters from the equator in the 

 depths and cool less saline waters from the 

 poles on the surface. 



3. Diastrophism has been periodic. 

 Viewed according to the periodicity of 

 diastrophism, the earth's history falls into 

 cycles, and each cycle into two periods, one 

 of inactivity and another of activity. The 

 periods of inactivity have been long, and 

 during a major part of the duration of any 

 such period the condition of inactivity has 

 been common to the entire surface of the 

 globe. Inactivity has not been cotermin- 

 ous, however, in different regions. 



The periods of diastrophic activity have 

 been relatively short, and as regards the 

 whole surface of the earth in general not 

 contemporaneous. The great ocean basins 

 are distinct dynamic provinces, and each 

 has experienced periods of diastrophic ac- 

 tivity peculiar to its individual history. 

 Orogenie districts are sharply limited by 

 local mechanical conditions. The epochs of 

 organic deformation are relatively brief. 

 And folding and unconformity, therefore, 

 are frequently not contemporaneous even 

 in one and the same dynamic province. 



4. The processes of erosion, sedimenta- 

 tion, chemical activity and organic evolu- 

 tion have been periodically conditioned ac- 

 cording to the periodicity of diastrophism. 

 The corresponding physical phenomena 

 (relief, deposits and climate) exhibit 

 rhythmic changes which repeat similar 

 conditions in like associations. Organic 

 forms, both faunas and floras, evolving 

 through intrinsic vital energy but not re- 

 peating, have been rhythmically condi- 

 tioned by changing environments. 



5. Erosion has been constant on land 

 surfaces through the activity of some of 

 the sub-processes, decay, denudation or ag- 

 gradation, which have never failed to make 

 a record. A fossil sub-aerial surface must 

 always show the record, unless it has been 

 obliterated. 



6. Marine sedimentation has sometimes 

 been inconstant. During periods of dias- 



