EVOLUTION 15 



cold, the utilization of available water or food with maxi- 

 mal efficiency, a larger number of young or, alternatively, 

 better care of the young and a longer breeding life, op- 

 timal co-operation within the group or between different 

 groups that can aid each other, and the exploitation of 

 new environmental possibihties that are not areas of 

 competition. 



Natural selection not only fosters the survival of new 

 forms with superior endowments but it drives new forms 

 into special niches of environment in which their unique 

 characters are most advantageous, and by this isolation 

 it prevents them from back-breeding with unmutated 

 forms, a process that could lead to the extinction of their 

 unique genetic characters. And when mutant forms are 

 isolated in special habitats, natural selection determines 

 what new combinations of genes and chromosomes will 

 be available for further mutation. 



Where once it seemed that random mutation and 

 natural selection could scarcely in an infinity of time fash- 

 ion the exquisite patterns of organic adaptation every- 

 where evident in nature, it is now accepted that, given 

 known or probable mutation rates as determined from 

 living animals and plants, or as judged by the rates of 

 appearance of new forms in the past, the evolution of 

 highly complex adaptations is not only a matter of high 

 statistical probabihty but one of relative rapidity. Mod- 

 em students of evolution see natural selection not as 

 blind but as creative and constructive. 



George Gaylord Simpson gives an analogy which, 

 though oversimplified, illustrates both the quantitative 

 aspects of modem evolutionary theory and the creative 

 force of natural selection itself: 



Suppose that you have in a barrel many copies of aU 

 the letters in the alphabet, and that you try, by drawing 

 three letters at a time, to spell the word 'cat,' discarding 

 all three letters every time you fail. You may never suc- 

 ceed, because you may draw all the cs, as or t's in 

 wrong combinations and discard them without ever hit- 



