es not affecting
their substantial identity gives rise to varieties. One species, then,
cannot become another, except by the assumption of a new specific
attribute, so that one species passes into another precisely as the
genus passes into the species, and that is just as, and not otherwise,
than one thought passes into another.

The fundamental law of the logical process is that we pass from the
generic towards the individual; from the simple to the complex.
Induction can proceed only by assuming a genus at the outset--that is,
by assuming certain attributes in the individual to be generic.
Translate this law into material forms, and we have each higher--that is
more complex--species evolved from the lower by the addition of some new
characteristic. This new attribute cannot be added by the functional
activity of the lower organism; that can only reproduce itself. A
thought does not change merely through repeated expression. We pass to
the conclusion of a syllogism, not from each term, but from a comparison
of the premises--and this requires an intellectual operation entirely
distinct from a mere apprehension of the terms. It is one thing to
comprehend the premises; it is quite another to deduce a conclusion from
them. It may necessarily follow, but it requires a separate act of the
mind to reach it. Premises will not of themselves reach a conclusion.

Reading this same truth in the forms of matter, we may say that species
will not pass into higher species without the intervention of a force
distinct from either. The impulse which adds a new attribute must be
intellectually separable from all those pre-existing, and its material
representation must be physically distinct from pre-existing forms. This
complete separability precludes the possibility of mere physical
genesis. The added attribute is presented by a new form of matter,
revealing the presence of a new thought--a new effect, requiring the
agency of a new cause. In accordance with the usual economy of nature,
who never duplicates her forces, change will be made only so far as may
be necessary to communicate the additional idea. Organisms representing
previous thoughts will be added to, in order to express the expansion of
the thought, instead of a creation _de novo_ in each instance. Thus an
identical cellular structure will be found in all organic beings, from
the lowest to the highest, each higher type carrying forward the idea
and its physical expression fo