If, now, inorganic masses, relatively so stable in composition, thus
have their outer parts differentiated from their inner parts, what must
we say of organic masses, characterized by such extreme chemical
instability?--instability so great that their essential material is
named protein, to indicate the readiness with which it passes from one
isomeric form to another. Clearly the necessary inference is that this
effect of the medium must be wrought inevitably and promptly, wherever
the relation of outer and inner has become settled: a qualification for
which the need will be seen hereafter.

* * * * *

Beginning with the earliest and most minute kinds of living things, we
necessarily encounter difficulties in getting direct evidence; since, of
the countless species now existing, all have been subject during
millions upon millions of years to the evolutionary process, and have
had their primary traits complicated and obscured by those endless
secondary traits which the natural selection of favourable variations
has produced. Among protophytes it needs but to think of the
multitudinous varieties of diatoms and desmids, with their
elaborately-constructed coverings; or of the definite methods of growth
and multiplication among such simple _Algae_ as the _Conjugatae_; to see
that most of their distinctive characters are due to inherited
constitutions, which have been slowly moulded by survival of the fittest
to this or that mode of life. To disentangle such parts of their
developmental changes as are due to the action of the medium, is
therefore hardly possible. We can hope only to get a general conception
of it by contemplating the totality of the facts.

The first cardinal fact is that all protophytes are cellular--all show
us this contrast between outside and inside. Supposing the multitudinous
specialities of the envelope in different orders and genera of
protophytes to be set against one another, and mutually cancelled, there
remains as a trait common to them--an envelope unlike that which it
envelopes. The second cardinal fact is that this simple trait is the
earliest trait displayed in germs, or spores, or other parts from which
new individuals are to arise; and that, consequently, this trait must be
regarded as having been primordial. For it is an established truth of
organic evolution that embryos show us, in general ways, the forms of
remote ancestors; and that the first changes und