cook: evolution through normal diversity 197 



species of plants and animals. The most familiar example 

 of heterism is the individual diversity of mankind, but the same 

 condition is recognized as soon as we become sufficiently familiar 

 with the members of other species of animals or plants. Di- 

 versity is reduced or eliminated temporarily by selective breed- 

 ing, or propagation in single or narrow lines, but reappears 

 when the natural condition of free interbreeding is restored. 

 Instead of tending to impede evolution, intercrossing of lines 

 of descent in species presents a condition most favorable for the 

 preservation and extension of new characters. The develop- 

 ment of multitudes of useless differences is the best evidence 

 of spontaneous development of useful characters. "The prime 

 motor in evolution," to use Meehan's words, "is an innate power 

 to vary, coexistent with the species itself, independent of any 

 conditions of environment." 



Laboratory geneticists may believe that species consist nor- 

 mally of uniform, identical individuals, or may suppose that the 

 members of species tend to become uniform or to remain uni- 

 form if placed under the same environmental conditions, but 

 these assumptions are not based on familiarity with natural 

 species. Theories may be projected and logical systems de- 

 duced from the assumption of uniformity, as though a world 

 of uniform species really existed, just as mathematicians follow 

 relations of symbols into space of four dimensions. Facts 

 are often obscured by elaboration of conventional ideas. Wider 

 application of biology in agriculture, eugenics, and sociology 

 awaits clearer perception and presentation of the underlying 

 evolutionary and environmental relations. 



