THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 43 



where he found ripple-marks, thin-leaved stalks of vegetation, obscure 

 insect tracks, and a series of sinuous convolute markings where some insect 

 burrowing beneath the burning sand had thrown up a long trail indistin- 

 guishable from worm tracks at the bottom of a shallow body of water. He 

 went over much of the area most carefully, certainly over far more than is 

 normally exposed in a geological outcrop, and utterly failed to find a single 

 criterion that would have prevented him from pronouncing the exposure 

 an old sea-bottom or flood-plain if it had been found fossil, and yet the 

 formation was going on before his very eyes on a sun-stricken bit of desert. 

 The insect burrows would have unhesitatingly been called worm tracks; 

 the insect tracks might have been made by any one of many aquatic forms 

 instead of beetles or grasshoppers; the vegetation once fallen and recorded 

 only as an imprint could not be told from a bit of aquatic vegetation. The 

 wind ripples upon most careful analysis might have revealed their origin, 

 but again, in the author's experience, sand collected in a delta deposit 

 has been pronounced dunesand which had drifted into the water. 



IX. CHECKS ON THE BIOLOGIST. 



Perhaps the greatest need by any worker in paleobiology is a compre- 

 hension of the nature of the movement of the land-masses. It is accepted 

 by the majority of geologists that certain portions of the earth's surface 

 have been dominantly land and elevated above the general level and that 

 others have been peristently depressed and occupied by oceanic waters, but 

 it is obvious to all that the lands have very frequently been covered by 

 shallow seas and that portions of the present ocean basin were once dry land. 

 That there has been law in the development of the present shape of the 

 continents there can be little doubt, but this law is yet to be discovered and 

 stated. The attention of paleobiologists is especially invited to the section 



following. 



(a) BRIDGES AND BARRIERS. 



The term "bridges" must be understood to include all possible means of 

 normal voluntary* movement by living forms of any kind between distinct 

 areas. Commonly we think of land connections between bodies of land, 

 but in the proper use of the term it must be applied to other conditions 

 channels between bodies of water, zones of climate, zones of equal altitude, 

 zones of similar vegetation, etc. anything which will permit migration or 

 interchange of life. The term "barriers" must be given the same free inter- 

 pretation. A "barrier" to some things will obviously be a "bridge" toothers 

 in many cases. 



The conflict between the paleobiologist and geologist, at least in the 

 present stage of both sciences, is largely confined to major questions of 

 communication, or connection between large masses of land and water. 

 The evidence of such "bridges" and "barriers" has largely been brought 



