CHANGING ANIMALS 11 



tion, mentioned in the preceding chapter. Darwin introduced the term 

 natural selection to convey the idea that nature exercises selection some- 

 what as an animal breeder does when he wishes to improve a stock of 

 domestic animals. The breeder selects as parents of the next generation 

 those individuals possessing qualities he wishes his stock to have. At the 

 same time he prevents the reproduction of those individuals which lack the 

 desired qualities. Thus selection by breeders (artificial selection) has two 

 aspects, one positive, the other negative. Similarly, natural selection is both 

 positive and negative in its working. 



Directing attention first to the negative aspect, we can readily under- 

 stand that if a bodily change is harmful, so that possessors of it are not so 

 well adapted to life as they would have been without it, the change will 

 be a handicap. Possessors of such a handicap may not live to maturity, or 

 if they do live they may not reproduce. Or if they do reproduce they 

 may not produce as large a proportion of the next generation as do their 

 unhandicapped brethren. As a result the harmful change will tend to dis- 

 appear in subsequent generations. 



This negative aspect of natural selection is important to animals as a 

 conservative or stabilizing force, insuring that undesirable changes are 

 weeded out and discarded from the species. Negative selection (stabiliz- 

 ing selection of Schmalhausen, 1949) helps to keep the species always at 

 its "adaptive peak" by preventing establishment of changes which would 

 lessen perfection of adaptation to the environment in which the species 

 lives. Negative selection is a preserver of the status quo; it is constructive 

 insofar as deviations from the established norm would be detrimental to 

 the species. But real progress is seldom achieved by enforcing conformity 

 to established patterns. Accordingly we look to the positive aspect of nat- 

 ural selection for the means of progressive change. 



If a bodily change is beneficial to its possessors, the latter will have an 

 advantage over their fellows who lack the change. If conditions are such 

 that competition is keen, this advantage may be sufficient to make a dif- 

 ference in ability to survive, or to produce offspring, or both. If it is, pos- 

 sessors of the change will produce more than "their share" of offspring. If 

 these offspring inherit the change, the result will be that among them will 

 occur a greater proportion of individuals possessing the change than pos- 

 sessed it in the parental generation. Let us suppose, for example, that 

 under a certain set of circumstances it is beneficial to an animal to have 

 long legs, the better to run away from enemies. If some members of the 

 species have longer legs (the result of mutation) than do others, the 

 longer-legged individuals may survive the ravages of their enemies better 

 than do the shorter-legged members. There will be a tendency for the 



