274 



GENERAL BIOLOGY 



shown in figure 168. Among the offspring of a single-spurred 

 and strongly bilateral flower 

 appeared this one plant bearing 

 mainly rive-spurred and radial 

 flowers. Such larger variations, 

 when they affect a number of 

 correlated characters so as to 

 change the aspect of the organ- 

 ism, and when with self fertiliza- 

 tion they are self maintaining 

 (i. e., when they "breed true"), 

 are known as mutations. That 

 mutants establish a new grade 

 of variations is evidenced by the 

 fact that each mutation estab- 

 lishes a new normal, about 

 which ordinary variations 

 fluctuate. 



Mutations appear rather rarely, and under conditions 

 that are not at present understood. Their importance as 

 starting points in ;;he development of new races of plants and 

 animals is well recognized by breeders. The long and care- 

 ful pioneer study of them by Hugo DeVries has made clear 

 their probable importance as starting points in the evolution 

 of new species. DeVries calls many of the mutants he has 

 found "elementary species." Their significance will again 

 be referred to in the chapter on inheritance. 



More young produced than can survive. The species of 

 organisms differ extraordinarily in the number of young 

 produced, but all afgree in the tendency to increase in a 

 geometric ratio. The offspring of a single parent may 

 number millions, or may be but few; but in either case, if 

 all survived to reproduce in like ratio, the earth would soon 

 lack standing room for the progeny. In the edge of the 



FIG. 168. The normal flowers of 

 Linaria. 



