94 



VARIATION 



Comment is hardly necessary to show the irregular working 

 of the animal machine, which in this case was a mature, strong, 

 and healthy cow. The per cent of fat varied all the way from 

 2.7 to 4.2 within the space of twenty-four hours. 



Influence of age upon functional activity. At birth the 

 vital functions and those of growth are at their maximum. At 

 this time growth seems to be proceeding with the " energy 

 of embryonic development." It continues at a maximum for a 

 time, 1 gradually declining in rate until maturity, when growth 

 of the general body ceases, although for certain parts (hair, 

 teeth, horn, hoof, reproductive cells, etc.) it continues nearly or 

 quite through life. In case of accident many parts not com- 

 monly indulging in growth after maturity are able to regenerate 

 with more or less success (bone, skin, blood vessels, nerves), 

 but among the higher organisms generally growth practically 

 ceases at maturity. 2 It is for this reason that feeding enterprises 

 are most profitable with young animals that are still growing. 

 At this age functional activity is greater and general bodily 

 efficiency higher. 



One reason why the lower grades of feeders are less econom- 

 ical producers than the better grades is that the animals are 

 older, the difference in age between fancy selected and inferior 

 steers being one to two years. 



Strength and endurance are evidently at their maximum 

 somewhat after maturity, although with the passing of that 

 age is also lost something of the power of rapid recuperation 

 and repair. 



The reproductive functions, undeveloped until a considerable 

 period after birth, attain a high degree of energy, if not their 

 maximum, somewhat before full maturity is reached. They then 

 decline, and fail entirely in old age. Their duration is therefore 

 considerably less than the life of the individual, often dropping 

 below 50 per cent of the full life period. 



1 The curve showing rate of growth at different ages has not been sufficiently 

 worked out, but enough has been done to indicate that the maximum rate of 

 growth is attained a few days after birth and that this maximum is never again 

 reached. 



2 Trees continue to increase in size, to some extent at least, during life, illustrat- 

 ing a marked difference between the higher plants and animals. 



