724 General and Applied Biology 



heights of human beings (Fig. 358), lengths of appendages, weights of 

 individuals, sizes of leaves, heights of trees of the same species. 



Discontinuous variations or mutations are the result of certain char- 

 acteristics, appearing abruptly in an individual, which are so distinctly 

 variable that they do not fit into the graded series of the main body. 

 (Mutations are more fully considered in the chapter on Heredity.) 



According to Their Direction. — Orthogenetic or determinate variations 

 follow such a sequence that there is a straight development along specific 

 and logical lines toward a definite goal. An example of this type is the 

 ancestry of the horse (Figs. 359 and 360) in which the five- toed ancestor 

 developed into the four-toed, then into the three-toed, and finally into 

 the one-toed persent-day form. 



76. 7a: n 73. 72. II. 70. 69. 68. 67. 66. 6i: 6f. 63i 62. 6/. 60. 5t SQ^ S7. 



HEIGHT IN INCHES 



Fig. 358. — Distribution curves showing heights of college men and women includ- 

 ing all ages; (— — — ) represents the men; ( ) represents the women. 



Fortuitous or indeterminate variations fluctuate back and forth about 

 a mean, apparently always within the same limits, generation after gen- 

 eration. For instance, the leaves of a tree may be larger one season than 

 another and may continue to vary back and forth several generations, 

 but they never fluctuate far from the mean size for leaves of a plant of 

 that particular species. Such variations are caused by ( 1 ) recombinations 



