ght to a standstill, not merely by specific
difficulties in the application of his theory, but also by a certain
instinctive or temperamental dissatisfaction with such a general
conclusion as the one I have indicated; and many able naturalists feel
the same difficulty to-day. Whether this be justified or not, it is
undoubtedly the fact that few working naturalists feel convinced that
the problem of organic evolution has been fully solved. One of the
questions with which research is seriously engaged is whether
variations or mutations are indeterminate, as Darwin on the whole
believed, or whether they may be in greater or less degree
determinate, proceeding along definite lines as if impelled by a _vis
a tergo_. The theory of "orthogenesis," proposed by Naegeli and Eimer,
makes the latter assumption; and it has found a considerable number of
adherents among recent biological investigators, including some of our
own colleagues, who have made important contributions to the
investigation of this fundamental question. It is too soon to venture
a prediction as to the ultimate result. That evolution has been
orthogenetic in the case of certain groups, seems to be well
established, but many difficulties stand in the way of its acceptance
as a general principle of explanation. The uncertainty that still
hangs over this question and that of the heredity of acquired
characters bears witness to the unsettled state of opinion regarding
the whole problem, and to the inadequacy of the attempts thus far made
to find its consistent and adequate solution.

Here, too, accordingly, we find ourselves confronted with wide gaps in
our knowledge which open the way to vitalistic or transcendental
theories of development. I think we should resist the temptation to
seek such refuge. It is more than probable that there are factors of
evolution still unknown. We can but seek for them. Nothing is more
certain than that life and the evolution of life are natural
phenomena. We must approach them, and as far as I can see must attempt
to analyze them, by the same methods that are employed in the study of
other natural phenomena. The student of nature can do no more than
strive towards the truth. When he does not find the whole truth there
is but one gospel for his salvation--still to strive towards the
truth. He knows that each forward step on the highway of discovery
will bring to view a new horizon of regions still unknown. It will be
an ill day for