though seasonal variations, clue to temporary 

 selections of more favorable combinations of 

 existing genes, still allow for a certain degree 

 of fluctuation. 



However, the stability of species cannot 

 withstand any long-enduring change in the 

 environment. Considered over a period of a 

 few years, or even a few centuries, most 

 natural environments — that is to say, envi- 

 ronments that are not greatly modified by 

 man's activities — remain fairly constant as 

 to the average of the cyclic changes of the 

 day and season. But considered over longer 

 periods, practically every environment re- 

 peatedly has undergone great changes. 



Geological Changes in the Environment. 

 The most obvious long-range environmental 

 changes are geological in nature. Even within 

 the period of human history — which is just 

 a moment in relation to the whole history of 

 the earth — certain regions of Asia that once 

 supported a flourishing civilization have be- 

 come deserts virtually devoid of life. Going 

 back some few thousand years further — still 

 within the existence of the human species — 

 vast regions of the earth, in North America, 

 Europe, and Asia, have several times been 

 subjected to arctic conditions due to glacia- 

 tion, while alternately these regions have 

 experienced tropical or subtropical climates. 

 And if we look still further back into geo- 

 logical time — millions of years — we see con- 

 tinents rising and sinking, changing their 

 contours and connections, and mountain 

 ranges elevated and worn down. In fact, 

 practically every region on the surface of the 

 earth, during the period in which life has 

 existed, has experienced virtually every kind 

 of environment: salt water, fresh water, high- 

 land, lowland, desert, swamps, under every 

 type of climate, ranging from arctic to tropi- 

 cal temperature (Fig. 28-5). 



These great physical fluctuations, while 

 most impressive, are not the only important 

 kind of change. The environment of a species 

 also consists of its relations to other organ- 

 isms; and whenever a species invades new 

 territory, increases or decreases in numbers, 



Natural Selection; Origin of Species - 545 



or changes its mode of life in any respect, 

 quite a number of other organisms are likely 

 to be affected directly or indirectly. Conse- 

 quently each significant modification of the 

 inanimate environment is always attended by 

 many changes in the animate environment 

 of all species in the affected region. 



Origin and Extinction of Species. Every 

 long-range change in the climate and other 

 environmental conditions has, owing to these 

 many factors, many effects upon the species 

 of the region: some species decline or even 

 become extinct; other species increase their 

 numbers and extend their range. Slight vari- 

 ations that were barely tolerated by the pre- 

 vious environment may assume real survival 

 value under the changed conditions, and 

 thus these particular variants will displace 

 the species types that dominated under the 

 old conditions. 



When an environmental change is favor- 

 able to a given species, its population will 

 increase, and during this period of less rigor- 

 ous competition a larger proportion and a 

 wider variety of offspring will be tolerated 

 for a number of generations. By chance some 

 of the new variants may be adapted to 

 slightly different environments, which en- 

 ables the species to extend its geographical or 

 its ecological range. When, however, en- 

 vironmental conditions become harder for 

 the species, many of its less fit or weaker 

 varieties may be wiped out, leaving just a 

 few sharply distinct varieties — or incipient 

 new species. In good times, generally speak- 

 ing, a species tends to increase in number and 

 variety, and greatly extends its range of terri- 

 tory. Then when hard times come, the spe- 

 cies may become extinct over large areas of 

 its former territory, leaving a few exception- 

 ally fit and perhaps very sharply different 

 varieties, each quite isolated from the others, 

 in different parts of the former range. This 

 factor of isolation, whether due to geograph- 

 ical separation or to other conditions, is very 

 important in the origin of species. Whenever 

 two varieties are isolated and prevented from 

 intercrossing for many generations, the muta- 



