NATURE OF CONTINUOUS VARIATION 525 



preserved specimens of each generation. At the same time he 

 also carefully observed plants in the field, and, when one was 

 found that showed extraordinary features, the experimental 

 method was applied to it. He was especially interested in vari- 

 ations, and his most notable contribution is on this subject. 

 He demonstrated by his painstaking work that there are two 

 kinds of variations — continuous or fluctuating and discontinu- 

 ous or saltative variations. Discontinuous variations are also 

 called mutations and are those extreme variations which sud- 

 denly arise and remain fixed, that is, they are transmitted to 

 succeeding generations. 



Nature of Continuous Variation. — Continuous variations are 

 the most common kind of variations. They are simply the fluc- 

 tuations that individuals show in size, shape, color, and other 

 characters. Thus red flowers vary in degree of redness, leaves 

 vary in shape and size, seeds vary in number per pod as well 

 as in shape and size, plants differ in height, shape, method of 

 branching, and so on. Continuous variations are chiefly due 

 to differences in the environment, such as differences in sun- 

 hght, food and water supply, temperature, and influences ex- 

 erted by one organism upon another. According to the work 

 of De Vries and other investigators, they are not inheritable 

 and, therefore, are constantly changing with the conditions that 

 cause them. They fluctuate around a mean or average which 

 remains practically constant, and, above or below this average, 

 the individuals varying gradually grow less in number as vari- 

 ability departs more and more from the average until a limit 

 in each direction is reached. Continuous variations follow the 

 law of Quetelet, the Belgian anthropologist, who found that 

 variability follows the law of probabihty. Small divergences 

 from the average are numerous, while larger ones are less numer- 

 ous, and the larger the less numerous they are. If, for example, 

 a bushel or any quantity of ears of Corn are separated into 

 piles according to length, there will be one length which will 

 include the greatest number and, above or below this length, 

 which is known as the average, the piles will decrease in size 

 as the length of ears in each pile are greater or less than the 

 average. This is well illustrated in Figure 469 in which 82 ears 

 of Corn with extreme lengths 4.5 and 9 inches are arranged in 

 10 piles according to size. The same fact is illustrated in Figure 



