44 WHITMAN. [Vou. Ii. 
incomprehensible nature of the force or forces contemplated. 
We see the effects only imperfectly, and are baffled in every 
attempt to understand the mode of action. For the present we 
must be content to search for the advection in which answers 
lie; and herein is found the chief value of theories. 
The more important speculations on this subject have 
taken the form of theories of heredity. In most of these 
theories, at least those of recent date, we find a fundamental 
idea which must be accepted as true ; namely, that the sexual 
cells reflect in some way in their chemico-physiological con- 
stitution all the typical structural features of the parent- 
organism. How all the hereditary tendencies can be contained 
in a single cell, and with such completeness that the developing 
organism repeats step for step the chief form-phases of a genea- 
logical history stretching through countless myriads of genera- 
tions, back from the present into the very dawn of life, and 
ultimately unfolds every detail of structure and feature of the 
parent-organism, is a mystery that transcends our understand- 
ing. The preformationists of last century took refuge in the 
celebrated inclusion (Einschachtelung, emboitement) theory, 
which made the real mystery unapproachable by hiding it be- 
hind an endless series of miracles. The triumph of the epigene- 
sists brought with it the reclamation of the problem, but left 
us with the indefinable ws essentialis of Wolff, the xzsus forma- 
tivus of Blumenbach. In more recent times we have seen vari- 
ous metaphysical hypotheses of extra-organic agents or forces 
supplanted” by physiological hypotheses which seek the cause of 
the phenomena in intra-organic forces. But the reaction which 
followed the fierce struggles with vitalism has left its impression 
on most of the theories now in vogue. Biologica] problems 
have been brought more and more under the influence of 
mechanical conceptions, which regard all phenomena from an 
objective standpoint. Science has vindicated this method, and 
as a method it is unassailable. It is no less indispensable to 
research in the organic than in the inorganic world; but the 
biologist is reminded at every turn that the method is not ex- 
haustive, and from the nature of the case it never can be. The 
biologist does not hesitate to follow the shibboleth of “ molecu- 
lar motion” to its final goal, but he must beware of being 
blinded by any artifice of method to distinctions which lie 
