378 



ANIMAL BIOLOGY 



ing the origin of the diverse inhabitants of the Earth. We now 

 realize that organic evolution is a bird's-eye view of the results 

 of heredity since the origin of life — the facts of inheritance 

 hold the key to the factors of evolution. Therefore we shall 

 consider the relations of recent discoveries in genetics to the 

 evolution problem — to the origin of the fitness of organisms. 



Selection. The process of selection has long been successfully 

 practiced by man to establish desirable types of domestic animals 

 and plants, and, as we know, Darwin assumed that a somewhat 

 similar but automatic selective process determines the survival 

 of the better adapted wild forms in nature. Darwin clearly 

 recognized that selection in itself can produce nothing — its efficacy 

 depends on the materials afforded by variation. But he did not 

 and, of course, could not make the modern sharp distinction be- 

 tween modifications, recombinations, and mutations. In general 

 he accepted all variations as at the disposal of selection, but em- 

 phasized the importance of small, finely-graded fluctuating varia- 

 tions in gradually producing, through many generations, a cumu- 

 lative effect in the direction of selection — variations that to-day 

 we know are, in part, modifications. 



The modern approach to the critical analysis of significant varia- 

 tions was opened by the work of two botanists, deVries and 

 Johannsen. DeVries laid stress on the importance of discontinuous 

 variations which he called mutations — a class of variations that 

 we have already discussed; while Johannsen made clear that in a 



homozygous germ complex, or 

 pure line, selection is ineffective, 

 as will appear beyond. 



Some of the problems of selec- 

 tion will be clear from an example. 

 Take, say, a quart of beans and 

 sort them into groups according 



to the weight of each bean. Then 

 Fig. 240. — Diagram to lllus- , 



trate a quart (population) of beans P ut ™™ group into a separate 

 assorted according to weight. cylinder and arrange the cylinders 



in a series according to the 

 weight of the enclosed beans. Now if we imagine a line connecting 

 the tops of the bean piles in the cylinders, it takes the form of a 

 normal curve of probability, or variability curve. A similar figure 



-w 



