1048 LIFE : OUTLINES OF GENERAL BIOLOGY 



Edwards. It concerns one of the Long-nosed Monkeys {Rhinopithecus 

 roxellance), which lives in companies in the high forests of Tibet, 

 where the snow lies for six months. It has a strong coat, though its 

 relatives in the tropical low grounds have not. The thick fur in the 

 snowy forest is doubtless adaptive, but this does not prove that it 

 arose as a transmissible modification. Long-haired varieties of 

 rabbits and guinea-pigs sometimes crop up apart from any known 

 climatic stimulus, and certainly apart from cold. 



On the whole we are forced to the conclusion that the evidence 

 of the heritability of climatic modifications is as yet very unsatis- 

 factory. 



Natural Acclimatisation in the Past. — All the world over 

 there are instances of nearly related species flourishing under 

 different climatic conditions, and few evolutionists have any 

 hesitation in regarding these as the outcome of divergent evolution. 

 Alpine flowering plants have often their counterparts in the valleys. 

 Shallow water marine animals are sometimes represented by related 

 species in the Deep Sea. Hundreds of such cases may be interpreted 

 as natural acclimatisations, and it may be recalled that while 

 Darwin did not think much of man's achievements in acclimatising, 

 he had no doubt as to Nature's powers in this direction. "We need 

 not, however, doubt that under nature new races and new species 

 would become adapted to widely different climates, by spontaneous 

 variation, aided by habits and regulated by natural selection." 



Three saving-clauses should be kept in mind, (a) When two 

 nearly related species are thriving in climatically different sur- 

 roundings, it should not be taken for granted, as it usually is, that 

 all their differences are now part and parcel of the inheritance. Some 

 of the differences may be modificational, hammered on each suc- 

 cessive generation in the course of development. There is need for 

 more experimental study of species, {h) There has been a tendency 

 to strain the interpretation of specific characters as adaptive to 

 particular conditions of life, such as those implied in climate. Many 

 characteristics separating related species in different localities may 

 be reasonably interpreted as climatic adaptations, but each case 

 should be carefully judged on its merits, (c) When a species is 

 extending its range in consequence perhaps of increasing numbers, 

 the factor of isolation may come into operation, say, in the form 

 of a river or a watershed, and variations may be separated off which 

 have no particular relation to the new territory or climate in which 

 the leaders of the advance find themselves. Thus new species may 

 arise by the physical segregation of diversely varying contingents 

 of an advancing army, till finally the climatic difference itself may 

 become an isolating factor. 



Accepting, with these saving-clauses, the idea of natural acclima- 

 tisation, we must now ask how it may have been effected. The 



