quite unintelligible why this strange succession of animal-like forms
appeared in the series at all. It had previously been generally
assumed that the man was found complete in all his parts in the ovum,
and that the development consisted only in an unfolding of the various
parts, a simple process of growth. This is by no means the case. On
the contrary, the whole process of the development of the individual
presents to the observer a connected succession of different
animal-forms; and these forms display a great variety of external and
internal structure. But WHY each individual human being should pass
through this series of forms in the course of his embryonic
development it was quite impossible to say until Lamarck and Darwin
established the theory of descent. Through this theory we have at last
detected the real causes, the efficient causes, of the individual
development; we have learned that these mechanical causes suffice of
themselves to effect the formation of the organism, and that there is
no need of the final causes which were formerly assumed. It is true
that in the academic philosophies of our time these final causes still
figure very prominently; in the new philosophy of nature we can
entirely replace them by efficient causes. We shall see, in the course
of our inquiry, how the most wonderful and hitherto insoluble enigmas
in the human and animal frame have proved amenable to a mechanical
explanation, by causes acting without prevision, through Darwin's
reform of the science of evolution. We have everywhere been able to
substitute unconscious causes, acting from necessity, for conscious,
purposive causes.* (* The monistic or mechanical philosophy of nature
holds that only unconscious, necessary, efficient causes are at work
in the whole field of nature, in organic life as well as in inorganic
changes. On the other hand, the dualist or vitalist philosophy of
nature affirms that unconscious forces are only at work in the
inorganic world, and that we find conscious, purposive, or final