EVOLUTION 1053 



(4) It is reasonable to suppose, though difficult to prove, that 

 change of climate in a country induced important changes of habit. 

 Thus Barrell and Lull have suggested that continental elevation 

 and consequent aridity, especially in the Himalayan region, led in 

 the Miocene or early Pliocene Ages to a dwindling of the forested 

 area where man's ancestors were at home. The alternatives were to 

 find other forests in warmer countries, as the present-day Anthropoid 

 apes did, or to be eliminated, or to come to earth and begin afresh 

 on a new line of life. The last solution may have been of critical 

 moment in the evolution of Hominoids. 



It is probable that climatic changes prompted the adoption of 

 new modes of life at many different levels among animals, for some 

 types became fossorial and other cursorial, some scansorial and 

 others aerial, some took to the water and others to the caves. 

 With the new habit and habitat would be associated new variations 

 and the evolution of fresh fitnesses. 



(5) The Lamarckian evolutionists maintain that functional modi- 

 fications, induced by novel habits, were (and are) hereditarily 

 entailed. Darwin admitted this possibility, and in connection with 

 acclimatisation he expressly says that "habit does something". In 

 the main, however, he relied on "spontaneous variation", by which 

 he meant, as he expressly says, that the novelties arise without 

 direct relation to the climate. 



Climatic changes in a country may also have played an important 

 part in punctuating the life-history. A kind of variation which has 

 not received adequate attention may be called "phasal". It includes 

 alterations in the tempo, or rate, or rhythm of metabolic processes, 

 or in the duration of particular phases in the life-history. In Verte- 

 brate animals, at least, this might be brought about by variations 

 (also, of course, requiring to be accounted for somehow) in the 

 secretory activity of the ductless or endocrinal glands, whose 

 hormones serve now as accelerators and again as brakes. The life- 

 histories of many types differ from one another in the shortening 

 or lengthening of particular arcs on the life-curve or trajectory. 

 Here is a kind of evolution to which climatic variations may have 

 applied a frequent spur. Thus when the rate of development was 

 such that the life-cycle could not be completed in the first summer, 

 there would be a tendency to favour variations in the direction of 

 interpolating a larval phase, as in insects, suited for an accumulation 

 of reserves, a reduced intensity of life in cold weather, a diminished 

 exposure of vulnerable surface, and so on. In the case of European 

 butterflies, for instance, comparatively few are able to survive as 

 adults; not very many pass the winter as eggs; a goodly number 

 pass the winter as encased pupse; but the majority, including most 

 of the phyletically older species, endure the winter as caterpillars. 

 Opinions may differ in regard to particular cases, but it is a legitimate 



