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 pupae 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 



