492 GENETICS IN RELATION TO AGRICULTURE 
although it might seem at first glance a very simple matter to account 
for the increased speed of the American trotter in successive generations 
by use inheritance, nevertheless in the light of modern conceptions of 
germinal constitution, the simplicity of this explanation is a snare and a 
delusion. Similarly the inheritance of milk- and butter-fat producing 
capacity in cattle and goats, the inheritance of the propensity to lay on 
flesh in meat-producing animals, and other characters of domestic animals 
of great practical importance simulate in the way they have been built 
up, it cannot be denied, the inheritance of acquired characters. But 
simulation is far from proof, and any attempt to examine the records 
from the standpoint of rigid proof cannot fail to impress the student with 
the unsatisfactory nature of the material involved. Practical breeding 
operations have been designed to produce results, not to prove or dis- 
prove any particular theory of inheritance. Consequently every vari- 
able which favors the production of results is made use of, so that it is 
only very rarely that a single variable occurs in a given set of practical 
data. As the number of variables increases it becomes more and more 
difficult to assess to each its particular value. To illustrate the diffi- 
culties of interpreting data such as we obtain from practical breeding 
operations, we need merely call attention to some of the important vari- 
ables which enter into such results, such as original germinal diversity, 
mutational changes, effects of selection, effects of functional modification, 
increasing knowledge of methods of developing animals, and maintenance 
of more favorable environmental conditions. The effect of all of these 
variable factors often enters into the end result in practical breeding 
operations. It is possible to determine statistically by means of rigid 
experimental analysis just how much is due to each one of them, but 
unfortunately this has not been done. For the present then we must 
conclude that it is a non-critical, unscientific attitude of mind which 
would assign to one of these variable factors, viz., the effects of func- 
tional modification, a leading importance in the end result, particularly 
when it is the most debatable one of all. Certainly we are in need of 
rigidly controlled experiments along this particular line. 
Parallel Induction.—It is a well-known fact that the germ cells are 
susceptible to injury under unfavorable conditions such as occur at times 
in the body. Accordingly under adverse conditions there is a possibility 
that the germ cells may be affected along with the body. The first 
experimental evidence definitely establishing this fact was obtained by 
Fischer, who subjected pupz of the moth, Arctia caja, to a low tem- 
perature and thereby produced a distinct new form with much darker 
wings, the males being darker in color than the females. By mating a 
pair of these 173 offspring were reared of which 17 resembled their parents 
in being much darker colored than the species type and again the males 
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