FUNCTIONAL VARIATION 91 



therefore, are better feeders than certain shorthorns, and there 

 are to be found among the latter breed individuals that are 

 more economical producers of milk than are many of the Jer- 

 sey breed. 



It has been said that no race was ever taken by a part-bred 

 horse over one that was racing bred. Whether literally true or 

 not, it is substantially correct, so intensely have the racing 

 ability and instinct been developed in certain breeds. 



These facts, together with what is known as to corn and beets, 

 show clearly that much yet remains to be done in the way of 

 developing functional activity, thereby increasing 



SECTION II VARIATION IN THE DEGREE OF ACTIVITY 

 OF NORMAL FUNCTIONS WITHIN THE SAME INDIVIDUAL 



Of no less scientific or economic interest than the data given 

 in the last section is the fact that functional variation is by no 

 means confined to differences between individuals. The prin- 

 ciple applies, though to a less extent, to the individual itself, 

 whose activities are not constant but variable from day to day 

 and throughout its life. 



Daily fluctuation in normal functions. It will be found upon 

 investigation that the ordinary functions of the body are unex- 

 pectedly irregular. Even the heart action is not absolutely 

 constant ; it is slower when the body is at rest than when it is 

 in action and is subject to great acceleration in certain diseases. 



All organs of the body work better some days than others ; 

 indeed, there is a distinct periodicity with each, a period of 

 increase, followed by one of maximum activity, and that again by 

 one of subsidence. This is the way organs rest. These periods 

 are evidently of different lengths, and it is therefore only occa- 

 sionally that the body as a whole is at its maxitmim. A good 

 example of this periodicity with functional deviation from day to 

 day is, again, that of milk secretion, which, as has been remarked, 

 is a kind of resultant of all the activities of the body. The fol- 

 lowing table gives the variations in the quantity and character 

 of milk from a single cow for a period of one month. 1 



1 Biilletin No. j/, Agricultural Experiment Station, University of Illinois. 



