438 ELEMENTARY BIOLOGY 



coloring and in shading, in the numbers of duplicated parts 

 (Fig. 232), and in physiological properties (Fig. 233). Examples 

 of all these kinds of variation are easily found. Examples of 

 physiological variation are the yield of milk, the proportions 

 of sugar or of some other component (Fig. 234), the amount 

 of sleep people need, relative immunity to infection, and so on. 



464. Causes of vari- 

 ation. We know that 



ZZZIZI^^ZIZ^^Z^IIZZZZZI^~ when a cow is under- 



nourished, she will not 

 yield as much milk as 

 she does when she is 



^ZZZZ^ZHZZZ^ properly fed and cared 



for. This accounts for 



FIG. 277. Variation in physiological , . , ,. 



characteristics much of the difference 



, between one farmer's 



Each line represents the relative amount of milk 



given by 16 cows in one month. The poorest yield COWS and his neighbor's 

 (represented by the shortest line) averaged 20 pounds Q ,, other 



a day ; the best cow averaged 30 pounds a day. Not 



only did one cow differ from another, but for each hand, in a given herd 

 cow the yield varied from day to day. In like man- f ,, f , . , 



ner, the percentage of fat in the milk varied from ' OWS ' ail 



cow to cow ; and for every cow, from day to day have received the Same 



care and feeding from 



the time they were born, there will still be great variations in 

 the ability to produce milk. In the first case we say that the 

 yield of the cow has been 'modified by the treatment she has 

 received. In the second case we say that the cows are of 

 different breeds , or strains. 



All around us we see examples of modifications resulting 

 from differences in the conditions of development. Differ- 

 ences of feeding affect plants as well as animals. In any 

 season we may see fields of stunted, backward crops and 

 fields of luxuriant growths. In every city we may see well-fed, 

 vigorous, and alert men and women, as well as shriveled, 

 miserable, and timid men and women. It is important to know 

 whether, and how far, these differences can be controlled. 



