e 
568 The American Naturalist. [July, 
edly, a greater difficulty in securing two qualities on a high 
plane of excellence, than to obtain an extraordinary perma 
ance in a single special direction. 
Milk and meat production are not strictly incompatable, 
and a high degree of excellence may doubtless be obtained 
with both. Greater skill is, however, required to combine the 
two qualities and retain them for any time, than to obtain a 
high development of either of them alone. A certain bal- 
ance, or equilibrium, in the expenditure of energy, must be 
secured in the general purpose animal, or there will be a ten- 
dency for some single quality to predominate. 
A tendency to the expenditure of energy in one direction 
during the period of growth, and in another direction when 
maturity is reached, may be cultivated and fixed by heredity- 
This principle is an important one for consideration in breed- 
ing dairy stock. When a cow is giving milk the tendency, or 
inherited habit of the organs of nutrition, may be to expend 
the entire energies of the system in the milk producing func- 
tion, and when she becomes “ dry,” the available energy may 
be expended in laying on fat. The difficulty is, however, to 
maintain a due balance of the two functions. If the fattening 
tendency predominates, the period of giving milk may be 
shortened and the activity of the function ultimately dimin- 
ished. One of the best precautions against this is to retain in 
perfection the milking type in the general form of the animal, 
and to keep up the milk secreting function as long as possible 
by proper management. Constant care in the selection and 
treatment of the animals will be required to secure the most 
desirable balance between the two functions, and prevent a 
predominance of either. 
EXERCISE AS A Factor IN IMPROVEMENT. 
From the general principles already noticed, it must be seen 
that the exereise of special organs, and of the general system, 
are necessary to secure the highest excellence in the working 
of the animal machine. We must keep in mind the fact that 
the exercise of an organ or group of organs, involves an 
expenditure of energy, and what is spent in one direction can- 
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