354 INTRODUCTION TO EVOLUTION 



this "struggle" what determines which individuals shall succeed, which 

 fail? 



We may answer this question by stating that those individuals will suc- 

 ceed which have favorable or advantageous inheritable variations of struc- 

 ture, physiology, and so on. Those individuals will fail which lack such 

 variations or which have unfavorable or harmful ones. In this statement 

 we have mentioned that the variations must be inheritable; while it is 

 true that noninheritable, favorable variations might enable an individual 

 to survive, such variations have no "future" so far as improvement of the 

 species is concerned. (See, however, the "Baldwin effect," pp. 420-425.) 

 The inheritable variations arise as new mutations and as new combina- 

 tions of genes originating in various ways (pp. 396-402). Darwin himself 

 placed great stress on the importance of variations, including individual 

 differences, and he recognized that to be useful in evolution they must be 

 inheritable. He was well acquainted with the fact that variation is uni- 

 versal, that "no two individuals are alike." In his day it was not known 

 to what extent these differences between individual and individual are in- 

 heritable, to what extent they are caused by environment and hence not 

 inheritable. 



The fact that many of the little variations in structure are not inherita- 

 ble was brilliantly demonstrated by the Danish geneticist, Johannsen. He 

 chose to work with the characteristic of weight, in beans. Taking advan- 

 tage of the fact that beans are self-fertilizing, he established a number of 

 pure lines, each descended from one bean. In general, pure lines de- 

 scended from heavy beans had greater average weight than did pure lines 

 descended from light beans. Since each pure line bred true to a certain av- 

 erage weight generation after generation, hereditary factors must have 

 been involved in the differences in weight. Since in any one pure line, de- 

 scended from one bean, the hereditary factors must have been identical 

 in all individuals, why were not all individuals identical in weight? Be- 

 cause superimposed upon the identical heredity were the effects of dif- 

 ferences in environment (in sunlight, moisture, food supply, etc., available 

 to individual plants and branches as they grew). When Johannsen took the 

 heaviest beans in a certain pure line and raised progeny from them he 

 found that the average weight was the same as that of the pure line itself, 

 or the same as he obtained when he took a light bean from the same 

 pure line and raised progeny from it. In brief, selection within a pure 

 line was not effective in producing change. Of the many implications of 

 Johannsen's work the one of most significance for us is the demonstration 

 that many observed variations in structure are produced by environment, 

 and that selection based on these environmentally induced variations does 



