his kind represent a return, in some measure, to earlier vitalistic
conceptions, but differ from the latter in that they are an outcome of
definite and exact experimental work. They are therefore often spoken
of collectively as "neo-vitalism."

It is not my purpose to enter upon a detailed critique of this
doctrine. To me it seems not to be science, but either a kind of
metaphysics or an act of faith. I must own to complete inability to
see how our scientific understanding of the matter is in any way
advanced by applying such names as "entelechy" or "psychoid" to the
unknown factors of the vital activities. They are words that have been
written into certain spaces that are otherwise blank in our record of
knowledge, and as far as I can see no more than this. It is my
impression that we shall do better as investigators of natural
phenomena frankly to admit that they stand for matters that we do not
yet understand, and continue our efforts to make them known. And have
we any other way of doing this than by observation, experiment,
comparison and the resolution of more complex phenomena into simpler
components? I say again, with all possible emphasis, that the
mechanistic hypothesis or machine-theory of living beings is not fully
established, that it _may_ not be adequate or even true; yet I can
only believe that until every other possibility has realty been
exhausted scientific biologists should hold fast to the working
program that has created the sciences of biology. The vitalistic
hypothesis may be held, and is held, as a matter of faith; but we
cannot call it science without misuse of the word.

When we turn, finally, to the genetic or historical part of our task,
we find ourselves confronted with precisely the same general problem
as in case of the existing organism. Biological investigators have
long since ceased to regard the fact of organic evolution as open to
serious discussion. The transmutation of species is not an hypothesis
or assumption, it is a fact accurately observed in our laboratories;
and the theory of evolution is only questioned in the same very
general way in which all the great generalizations of science are held
open to modification as knowledge advances. But it is a very large
question what has caused and determined evolution. Here, too, the
fundamental problem is, how far the process may be mechanically
explicable or comprehensible, how far it is susceptible of formulation
in physico-chemical