physico-chemical phænomena on the one hand, and vital phænomena on the
other. At present, however, we assuredly know of none; and I think we
shall exercise a wise humility in confessing that, for us at least, this
successive assumption of different states--(external conditions
remaining the same)--this _spontaneity of action_--if I may use a term
which implies more than I would be answerable for--which constitutes so
vast and plain a practical distinction between living bodies and those
which do not live, is an ultimate fact; indicating as such, the
existence of a broad line of demarcation between the subject-matter of
Biological and that of all other sciences.

For I would have it understood that this simple Euglena is the type of
_all_ living things, so far as the distinction between these and inert
matter is concerned. That cycle of changes, which is constituted by
perhaps not more than two or three steps in the Euglena, is as clearly
manifested in the multitudinous stages through which the germ of an oak
or of a man passes. Whatever forms the Living Being may take on, whether
simple or complex, _production_, _growth_, _reproduction_, are the
phænomena which distinguish it from that which does not live.

If this be true, it is clear that the student, in passing from the
physico-chemical to the physiological sciences, enters upon a totally
new order of facts; and it will next be for us to consider how far these
new facts involve _new_ methods, or require a modification of those with
which he is already acquainted. Now a great deal is said about the
peculiarity of the scientific method in general, and of the different
methods which are pursued in the different sciences. The Mathematics
are said to have one special method; Physics another, Biology a third,
and so forth. For my own part, I must confess that I do not understand
this phraseology.