44 THE PROBLEMS OF EVOLUTION 



The species must embrace all of these forms and 

 the many others which appear as minor variations. 

 It is characterized for our convenience by the lim- 

 ited assemblage of characters common to all indi- 

 viduals, but it consists in reality of every character 

 produced by all individuals, wherever they may 

 occur in its entire range. 



Obviously the intricacy of this association of 

 many individuals, widely scattered, with an ex- 

 treme variety of environmental conditions, offers 

 an opportunity for the development of almost 

 infinite variety in the species. A high degree of 

 constancy is not impossible; some species vary to 

 a scarcely appreciable extent, no matter how wide 

 their dispersal and the variability of the conditions 

 to which they are exposed. But within our powers 

 of observation a species may be very constant as 

 a rule and still include many individual variations 

 from time to time, apparently because the proper 

 association of heritage and environment for their 

 production is rarely attained. There is a possibility 

 even that existing species may encounter conditions 

 with which they have never before come into con- 

 tact. Such things have happened. We are told by 

 geologists that the evolution of the horse was ac- 

 companied by increasing dryness in North America 

 and that the onset of the glacial periods changed 

 the entire climate of this continent. Granting the 

 ability of the organism to live in spite of the new 



