THE STRUGGLE FOR EXISTENCE. 75 



We have already seen (p. 34) that when dealing with 

 large numbers of individuals most characters conform to 

 this rule and vary continuously. On the other hand, 

 discontinuous variation would give rise not to a graduated 

 curve but to a series of steps or jumps, from one stage to 

 the next above. A tendency towards discontinuity is 

 shown in "meristic" variation. Thus, a cell will either 

 remain single or divide into two, the antenna of an insect 

 may have four, five, or six joints, the number of dorsal 

 vertebrae in a mammal may vary from twelve to thirteen, 

 or from thirteen to fourteen, and so on. But even 

 meristic variations need not be discontinuous, and seldom 

 is gradation more perfectly shown than in the variation 

 of segmental nerves in the plexus supplying the limb of 

 a vertebrate (pp. 93, 107). 



The whole subject has been greatly confused by the 

 failure to distinguish between continuity in the variation 

 of characters and continuity in the changes of hereditary 

 factors. Modifications vary continuously, because the 

 incidence and quality of the factors of the environment 

 are due to chance. Mutations may be discontinuous if 

 the changes in the factors giving rise to them are sudden 

 and large enough. But the variation of characters due 

 to mutation may also be more or less continuous, owing 

 for instance to their reactions being modified by other 

 factors, as in incomplete dominance (p. 49). Again an 

 apparently simple character may become more and more 

 pronounced when it is really a complex character, de- 

 pending on the co-operation of a number of separate 

 factors reinforcing each other as with the red colour of 

 wheat studied by Nilsson-Ehle, which develops only to 

 its full extent when three factors are all present. The 

 gradation in the mean weight in the different strains of 

 the beans already so often mentioned (pp. 45, 53, 71) is 

 perhaps also due to the co-operation of several factors. 

 Moreover, it is possible that the factors themselves may 

 show grades of strength and development ; certainly if 



