322 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
ment. However, it is still a question whether the material used may 
not be heterogeneous as regards the germinal factors that condition 
certain physiological characters. 
Stockard’s investigations on the effect of alcohol on the progeny 
of guinea pigs have shown that the germ cells as well as the somatic 
tissues of the alcoholized animals are injured. 
On the whole it must be admitted that the experimental induction 
of heritable variations is still largely an unworked field. The complex 
conditions to be considered and consequent obstacles to be overcome 
are appreciated by no one more fully than by those who have at- 
tempted such investigations. For, as Tower has said: ‘‘It is evident 
that the problem of germinal change is one of difficulty, and involves 
more of indirect than of direct methods of investigation. There is 
little reason to expect that present biochemical methods can give a 
solution, but they may give valuable suggestions for further indirect 
investigation. It seems not improbable, however, that this problem, 
like so many others in biology, must await the solution of the larger 
question of what life is before it will be possible to express in exact 
terms the nature of germinal changes. Our present status, with 
several methods of production and much knowledge of the behavior 
of induced germinal changes available, is a basis from-which great 
advances in knowledge and in operation may reasonably be expected.” 
