Jantjaey 27, 1922] 



SCIENCE 



89 



revealed a slow but stately progression from 

 smaller to larger continents, from more active 

 mountain-building and volcanic action to 

 quieter conditions, and of warmer, moister 

 climates to colder and drier climates, these 

 results of more recent studies are particularly 

 instructive. The earth's physical history, dur- 

 ing the past fifteen or more geologic periods, 

 seems actually to be represented by neither a 

 perceptibly ascending nor a descending curve, 

 but by a slightly wavy line which is either 

 essentially horizontal or more probably is 

 rising or descending so very slowly that all of 

 the recognized periods of the earth's history 

 combined are too short to show the trend. 



When we turn to the history of living things 

 on the earth, however, we have unrolled before 

 US a definite progression which we are accus- 

 tomed to regard as an upward trend. 



Logically, geologists should not include the 

 history of organisms as a part of their science. 

 Such matters should concern the zoologist and 

 botanist. Prom the outset, however, geologists 

 have been much interested in the study of 

 fossils and the biologists have seldom protested 

 against this adoption. The study of fossil 

 organisms has indeed absorbed nearly all the 

 attention of many men generally regarded as 

 geologists. 



No question related to the earth's history 

 arouses greater interest than that of the origin 

 of life. Yet of the beginnings of life we have 

 absolutely no trace in the geologic record. Our 

 notions regarding that remarkable event are 

 necessarily derived from speculations based 

 on the known laws of chemistry and physics, 

 and on the theories of evolution. 



Land plants of such familiar types as the 

 ferns and club-mosses we have traced by their 

 fossil remains back to the Devonian or even to 

 the Silurian period. Ages before that, at least 

 as far back as the early part of the Algonkian 

 period, the sedimentary deposits were of such 

 a nature as to strongly indicate that the land 

 was in part covered by an effective mg,ntle of 

 vegetation. This may have been supplied by 

 the mosses and lichens, such as now form that 

 thick and characteristic blanket of the ground 

 in the sub-arctic regions — the "tundra." They 

 would effectively cover the ground, but only 



under the most favorable circumstances could 

 they be preserved as fossils. Aquatic alga3 

 have been satisfactorily identified in consider- 

 able variety in rocks that were laid down early 

 in the Proterozoie era; and there are indirect 

 evidences and perhaps the actual fossil re- 

 mains of bacteria quite as early. On pui-ely 

 theoretical grounds this has long been ex- 

 pected. 



Animals of comparatively advanced classes, 

 such as the arthropods, were highly diversified 

 as far back as the lower Cambrian epoch and 

 have been reported from even older rocks. 

 Simpler animals, such as the radiolarians, 

 have been detected in the pre-Cambrian rocks 

 of France. "We may, therefore, reasonably 

 expect to find fossil animals in even the oldest 

 sedimentary rocks, if they are not so altered 

 by metamorphism as to have lost their original 

 structures. 



Under the best of circumstances, however, 

 we can hardly hope to find traces of animals 

 more primitive than those represented among 

 groups already known in fossil and living form. 



No branch of geologic history has been so 

 assiduously studied as the evolution of the 

 animals. Thanks to this activity, ranging 

 over more than a century, we now have a 

 large fund of information from which to gen- 

 eralize. It has long been recognized that new 

 genera, orders and classes have made their 

 appearance one after another, risen to prom- 

 inence, flourished for a while, and then de- 

 clined either to extinction or to a lowly status 

 where some of them have lingered on in ob- 

 scurity through many succeeding periods. The 

 ammonites, which once inhabited every sea on 

 the globe in great variety, disappeared utterly 

 at the close of the Mesozoic era, and to-day 

 we have nothing more closely related to them 

 than their cousin, the Nautilus. The great 

 class of reptiles, which dominated the land in 

 the Mesozoic era, and whose sway was then 

 apparently unchallenged by any other living 

 thing, has largely disappeared and such types 

 as have survived to the present day have been 

 relegated to the jungles and waste places of 

 the earth, where they must depend for the 

 most part on concealment or upon fleetness to 

 escape from their more powerful enemies. 



