this take place?" the chief new idea would be, the idea of adaptation to
purpose,--the notion, that the constituents of animal bodies are not mere
unconnected parts, but organs working together to an end. Let us consider
the tail of the lobster again from this point of view. Morphology has
taught us that it is a series of segments composed of homologous parts,
which undergo various modifications--beneath and through which a common
plan of formation is discernible. But if I look at the same part
physiologically, I see that it is a most beautifully constructed organ of
locomotion, by means of which the animal can swiftly propel itself either
backwards or forwards.

But how is this remarkable propulsive machine made to perform its
functions? If I were suddenly to kill one of these animals and to take
out all the soft parts, I should find the shell to be perfectly inert, to
have no more power of moving itself than is possessed by the machinery of
a mill when disconnected from its steam-engine or water-wheel. But if I
were to open it, and take out the viscera only, leaving the white flesh,
I should perceive that the lobster could bend and extend its tail as well
as before. If I were to cut off the tail, I should cease to find any
spontaneous motion in it; but on pinching any portion of the flesh, I
should observe that it underwent a very curious change--each fibre
becoming shorter and thicker. By this act of contraction, as it is
termed, the parts to which the ends of the fibre are attached are, of
course, approximated; and according to the relations of their points of
attachment to the centres of motions of the different rings, the bending
or the extension of the tail results. Close observation of the newly-
opened lobster would soon show that all its movements are due to the same
cause--the shortening and thickening of these fleshy fibres, which are
technically called muscles.

Here, then, is a capital fact. The movements of the lobster are due to
muscular contractility. But why does a muscle contract at one time and
not at another? Why does one whole group of muscles contract when the
lobster wishes to extend his tail, and another group when he desires to
bend it? What is it originates, directs, and controls the motive power?

Experiment, the great instrument for the ascertainment of truth in
physical science, answers this question for us. In the head of the
lobster there lies a small mass of that peculiar tissue which