1897.] The Biologie Origin of Mental Variety: 5 
mediate or immediate, are here certain, and make the possibil- 
ity of their being involved in other end-organ processes the 
more likely, and, therefore, the more confusing. The question 
whether temperature acts directly on the nerves of heat and 
cold is again illustrative of the difficulties of our end-organ 
problem. The wide diversity of stimuli that, apparently, 
affect the pain nerves is also perplexing. And finally the fact 
that all sorts of artificial stimulation of the cut stumps of the 
sensory nerves—pricking, pressing, burning, and the applica- 
tion of ice, electricity and various chemicals—alike produce 
the one customary effect, sets the task of deciding definitely 
as to the initiation of sensory impulses entirely in the future. 
While it is thus impossible for us to determine the nature of 
the end-organ processes by positive evidence of their present 
condition, there still remains the wide field of their morphol- 
ogy, which ought, if we could understand it, to reveal both 
their nature and their evolutionary history. But here again 
we find ourselves among uncertainties. The best we may do, 
therefore, in survey of the final hypotheses among which 
future science must find the ultimate truth, is to set in order 
the loose-ends of their several, at present, indeterminable possi- 
bilities, somewhat categorically, and with brief enumeration © 
of the conditions involved in each. In so doing, we reach the 
limits of what should be expected regarding these matters 
within the limits of this entirely prospective paper. 
As a step to this end, it is necessary to recognize still one 
other source of difficulty. While all problems of physiological 
morphology are much complicated by uncertainty whether 
we must be guided therein by Weismann or by Lamarck, we 
must anticipate peculiar difficulties from this source, in our 
present problem. We become aware of this the moment we 
weigh our two main propositions in view of these rival biologic 
principles. If life began with one primary sense and developed 
our various ones at successive periods, the cue for this develop- 
ment would be different under the one theory than under the 
other. If Weismann is to be followed we must depend chiefly 
on spontaneous variations; in which case we must estimate the 
difficulties of new specific energies being born into the central 
