the Study of Variations, 1894) to be quite frequent in occur- 

 rence and by DeVries and others to be of great importance 

 in the development of new species. Such variations are 

 termed ?nutations by DeVries, and will be discussed at 

 greater length under the head of Mutation Theory. 



Kinds of Variations.— In the study of these two classes 

 of variations four different kinds are now recognized: 



1 . Morphological, relating to differences in form or size : 



(a) Variations in size, of apples, horses, sheep, etc. 



(b) Variations in relative proportions of parts, e.g. , 

 wider face, longer legs, etc. 



2. Substantive, relating to differences in quality of the 



structure : 



(a) Variations in fineness of bone, firmness of 

 muscles, etc., of animals. 



(b) Variations in quality of milk, brain, etc. 



(c) Variations in quality of apples, sugar beets, 

 corn, etc. 



(d) Variations in color, immunity to disease, 

 hardiness, etc. 



3. Meristic, relating to deviations in pattern, expecially 



to repeated parts: 



{a) Variations in symmetry, either radial or bi- 

 lateral, in double flowers, in four-leafed clovers. 



{b) Extra teats in cows, extra fingers or toes in 

 man., etc., etc. 



4. Functional, relating to deviations in the normal 

 activity of the various organs and parts of the body 

 or the plant : 



(fl) Variation in the degree o' activity of normal 

 functions between different individuals of 

 same species, e.g., in milk secretion, meat pro- 

 duction, speed, resistance to disease, and vital- 

 ity generally. 

 ib) Variations within the same individual, e.g. 

 daily fluctuations, influence of age, use and 

 disuse, and food upon functional activity. 

 ic) Modification of normal functions by external 



or other influences, e.g., galls, etc. 



id) Normal functions exercised under abnormal 



conditions. 



Curves of Variation.— Tho. graphic representation of 



variations is often both instructive and suggestive. Use is 



made of "graph" paper (paper ruled into fine squares) in 



the plotting of "frequency curves." The scale used in 



plotting will depend on the number of cases to be covered. 



24 



