976 LIFE : OUTLINES OF GENERAL BIOLOGY 



Lull's thesis is the able development of an old but hitherto in- 

 sufficiently clear idea — at first even creationist, as with Cuvier — 

 that the great advances and retrogressions may be correlated with 

 geologically demonstrable changes in the physical environment — in 

 climate and in continental elevations in particular. To take the 

 simplest case, it is plain that the setting in of an Ice Age — which has 

 happened over and over again in the course of the earth's history — 

 would involve severe sifting, and on the whole, it would seem, re- 

 tardation. "As the physician, by a clever device, can record graphi- 

 cally the pulsations in the blood stream which are synchronous with 

 the throbbing of the human heart, so I have drawn a curve to show 

 the correspondence between the pulse of life and the heavings of 

 the earth's broad breast." This is a thesis of great interest; so we 

 outline a few of Lull's illustrations; and may then add a note on the 

 probability of there being internal factors which co-operate with 

 the undeniable external factors in determining the pulse of organic 

 evolution. In regard to the illustrations, it may be that some remain 

 dubious ; but that wiU not affect the validity of the general idea that 

 "the expression-points of evolution are almost invariably coincident 

 with some great geologic change". 



Each of the great colonisations of the dry land by animals from 

 the waters has been a momentous event in organic evolution. The 

 Worm invasion led on to soil-making; the air-breathing Arthropod 

 invasion led on to the linkage between insects and flowers ; the Verte- 

 brate invasion by adventurous fishes led on, through Amphibians, 

 to Reptiles, and thence to Birds and Mammals. In the great majority 

 of cases the path of the invaders or colonists was from rivers and 

 lakes, swamps and marshes, on to dry land, though the starting- 

 point may usually have been the sea. Now it seems reasonable to 

 suppose with Lull that the external impelling cause of the vertebrate 

 emergence, which led through lung-fishes to Amphibians, was 

 aridity. "Diastrophic movement during the Silurian Period initiated 

 a widespread aridity which culminated in the latter part of the 

 period, continued with varying intensity into and through Devonian 

 time, and rose again to greater severity in the latter part of that 

 period." Rivers and lakes would shrink, and there would be un- 

 comfortable crowding in the pools; some types, like the African 

 mudfish, would save themselves by sinking into quiescence during 

 the dry season; others would creep ashore and learn in various 

 ways to breathe dry air; many, of course, would be eliminated. 

 Drought is often a drag on life, for one cannot forget that more 

 than eighty per cent, of living matter is water; yet indirectly it may 

 be a spur, by prompting to such achievements as getting on to dry 

 land and learning to breathe by the internal surfaces of lungs. 



But oscillations and alternations seem to have been very character- 

 istic of the history of climates; and in part of the Carboniferous 



