384 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1910. 



far into the undeciphered eras which preceded the readable record, 

 and no one to-day can safely affirm the precedence of either over the 

 other on the basis of the physical record, either in time or in genesis, 

 whatever his theoretical leanings may be. 



Among the agencies assignable for the extension of the land are 

 such as deform the earth and by deepening its basins and increasing 

 its protrusions draw the water into the deeps and give relief and 

 extent to the land. Among the agencies that make for the extension 

 of the sea are the decay and erosion of the surface of the land and 

 the girdling cut of the waves about its border. By the unceasing 

 work of these gentle, but persistent, agencies, the land is brought 

 low and the sea creeps out upon its borders. If the deforming of the 

 earth body were held in abeyance for an indefinite period, the lower- 

 ing of the land, the filling of the basins by the inwash and the 

 spreading of the sea would inevitably submerge the entire surface of 

 the globe and bring an end to all land life. Great progress in such 

 sea transgression took place again and again until perhaps half the 

 land was submerged, but before land life was entirely cut off or 

 even very seriously threatened a regenerative movement in the body 

 of the earth took place, the land was again protruded and extended 

 and the sea again restricted. 



Here, then, also, there have been a series of reciprocal movements 

 which, while they have brought alternate expansions of land life 

 and of sea life, have notwithstanding conduced to the preservation 

 of both under shifting stimulating conditions, and have thus main- 

 tained the continuity of the two great divisions of life, if indeed 

 they have not promoted the evolution of both by alternate stress and 

 tension. 



It appears, then, in the large view that in each of the great groups 

 of terrestrial conditions on which life is dependent, there has run 

 through the ages, vast as they have been, a series of oscillatory move- 

 ments that have brought profound changes again and again, but 

 which have never permitted any of the disasters that seemed to be 

 threatened by these movements to go far enough to compass the gen- 

 eral extinction of life. These reciprocal movements seem to be 

 dependent upon a balancing of the actions of the opposing agencies 

 that has the aspect of a planetary equilibrium. It does not seem to 

 me too much to regard it as an automatic regmlative system. A 

 clear insight into the intimate workings of the complex of agencies 

 that cooperate in this regulative system is rather a task of the future 

 than an attainment of the present, and I am not now justified in 

 offering more than suggestions of what may prove to be among the 

 main features of the system, in the hope that you will receive them 

 with due reserve. 



