THE UTILIZATION OF ENERGY. 519 
As these figures show, No. II was decidedly inferior to No. IIL 
in walking without load. In trotting, on the other hand, he was 
somewhat the superior of No. III, or in other words the change 
from walking to trotting caused much less increase in his metabo- 
lism. Like No. XIII, he carried a load with decidedly less expendi- 
ture of energy than did No. III. For the forms of work in which 
the percentage efficiency could be measured the results were as 
follows, the grades, however being not exactly the same for No. II 
as for No. III: 
Horse No. II, Horse No. III, 
Per Cent. Per Cent. 
Ascending, moderate grade... 33.2 34.3 
fs heavier grade..... 31.7 33.7 
Draft, nearly horizontal...... 29.0 31.3 
«up a grade............ 22.4 22.7 
It seems a fair presumption that such individual differences 
as those above instanced are caused, in large part at least, 
by differences in the conformation of the animals resulting from 
heredity or “spontaneous” variation. A strain of horses which 
has been bred and trained especially for the saddle through a 
number of generations might very naturally be expected to be 
more efficient in carrying a load than a strain which has been bred 
for speed in harness or strength in draft, while the latter might as 
naturally excel the former in efficiency at the trot or in draft. 
Similarly, a race of horses developed in a hilly country might be 
expected to be more efficient in ascending a grade than one in- 
habiting a flat region. It would seem, too, that these differences 
may be not inconsiderable. The results cited suggest an interest- 
ing line of thought and investigation for the student of breeding. 
TRAINING AND FaTIGuE.—It is a familiar experience that any 
unaccustomed form of work is much mere fatiguing at first than it 
is later. This is due in part to the fact that in making unfamiliar 
motions more accessory groups of muscles are called into activity 
than are necessary later when more skill has been acquired. The 
experience of a learner on the bicycle is an excellent example of 
this. In the second place, however, simple exercise of a group of 
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