of the most
enigmatical cases known, namely, the regeneration of the lens of the
eye in the tadpoles of salamanders. If the lens be removed from the
eye of a young tadpole, the animal proceeds to manufacture a new one
to take its place, and the eye becomes as perfect as before. That such
a process should take place at all is remarkable enough; but from a
technical point of view this is not the extraordinary feature of the
case. What fills the embryologist with astonishment is the fact that
the new lens is not formed in the same way or from the same material
as the old one. In the normal development of the tadpole from the egg,
as in all other vertebrate animals, the lens is formed from the outer
skin or ectoderm of the head. In the replacement of the lens after
removal it arises from the cells of the iris, which form the edge of
the optic cup, and this originates in the embryo not from the outer
skin but as an outgrowth from the brain. As far as we can see, neither
the animal itself nor any of its ancestors can have had experience of
such a process. How, then, can such a power have been acquired, and
how does it inhere in the structure of the organism? If the process of
repair be due to some kind of intelligent action, as some naturalists
have supposed, why should not the higher animals and man possess a
similar useful capacity? To these questions biology can at present
give no reply. In the face of such a case the mechanist must simply
confess himself for the time being brought to a standstill; and there
are some able naturalists who have in recent years argued that by the
very nature of the case such phenomena are incapable of a rational
explanation along the lines of a physico-chemical or mechanistic
analysis. These writers have urged, accordingly, that we must
postulate in the living organism some form of controlling or
regulating agency which does not lie in its physico-chemical
configuration and is not a form of physical energy--something that may
be akin to a form of intelligence (conscious or unconscious), and to
which the physical energies are in some fashion subject. To this
supposed factor in the vital processes have been applied such terms as
the "entelechy" (from Aristotle), or the "psychoid"; and some writers
have even employed the word "soul" in this sense--though this
technical and limited use of the word should not be confounded with
the more usual and general one with which we are familiar. Views of
t