others, have attacked the evolution of functions.

It will be the task of some future physiologist to engage in the study
of the evolution of functions with the same zeal and success as has
been done for the evolution of structures in morphogeny (the science
of the genesis of forms). Let me illustrate the close connection of
the two by a couple of examples. The heart in the human embryo has at
first a very simple construction, such as we find in permanent form
among the ascidiae and other low organisms; with this is associated a
very simple system of circulation of the blood. Now, when we find that
with the full-grown heart there comes a totally different and much
more intricate circulation, our inquiry into the development of the
heart becomes at once, not only an anatomical, but also a
physiological, study. Thus it is clear that the ontogeny of the heart
can only be understood in the light of its phylogeny (or development
in the past), both as regards function and structure. The same holds
true of all the other organs and their functions. For instance, the
science of the evolution of the alimentary canal, the lungs, or the
sexual organs, gives us at the same time, through the exact
comparative investigation of structure-development, most important
information with regard to the evolution of the functions of these
organs.

This significant connection is very clearly seen in the evolution of
the nervous system. This system is in the economy of the human body
the medium of sensation, will, and even thought, the highest of the
psychic functions; in a word, of all the various functions which
constitute the proper object of psychology. Modern anatomy and
physiology have proved that these psychic functions are immediately
dependent on the fine structure and the composition of the central