The Decisive Experiment 175 
for Weismann’s amputation which can prove nothing. 
Similarly light hammering, continually repeated, which 
a proper mechanism might automatically perform upon 
certain parts of the skull of hornless animals, would be 
better than cutting off or breaking the horns in horned 
animals, 
All these artificial stimuli would certainly produce in 
each individual the hypertrophy of the organ upon which 
they act. It could then be seen whether the repetition of 
these stimuli throughout a series of generations would be 
followed by the production of individuals in which these 
organs would possess at birth even in a small proportion 
the greater development that had been acquired in several 
successive generations of its ancestors. The performance 
of such experiments upon guinea pigs or rats would not 
seem to present very great practical difficulties; never- 
theless, so far as we know, it has never yet occurred to 
any one to make them or to propose them. 
But in all these experiments one must never forget 
that it is just the littleness of the inheritable fraction of 
an acquired quantitative variation, that constitutes the 
great difficulty of verification of the Lamarckian principle. 
Galton proposes as is known to select for experi- 
ment rather the inheritance or non-inheritance of certain 
acquired instincts. He advises for example to adopt the 
method of the following experiment of Mobius upon the 
pike; Mobius divided a large glass receptacle into two 
compartments by means of a perfectly transparent glass 
septum and placed the pike in one compartment and in 
the other little gudgeons upon which the pike usually 
feeds. It followed that whenever the pike precipitated 
himself toward any of the little fishes he was stopped by 
the glass against which he hit. After several weeks of 
