law is that
each plant or animal produces others of like kind with itself: the
likeness of kind consisting not so much in the repetition of individual
traits as in the assumption of the same general structure. This truth
has been made by daily illustration so familiar as nearly to have lost
its significance. That wheat produces wheat,--that existing oxen are
descended from ancestral oxen,--that every unfolding organism ultimately
takes the form of the class, order, genus, and species from which it
sprang; is a fact which, by force of repetition, has assumed in our
minds the character of a necessity. It is in this, however, that the law
of hereditary transmission is principally displayed; the phenomena
commonly named as exemplifying it being quite subordinate
manifestations. And the law, as thus understood, is universal. Not
forgetting the apparent, but only apparent, exceptions presented by the
strange class of phenomena known as "alternate generation," the truth
that like produces like is common to all types of organisms.

Let us take next a universal physiological law of a less conspicuous
kind. To the ordinary observer, it seems that the multiplication of
organisms proceeds in various ways. He sees that the young of the higher
animals when born resemble their parents; that birds lay eggs, which
they foster and hatch; that fish deposit spawn and leave it. Among
plants, he finds that while in some cases new individuals grow from
seeds only, in other cases they also grow from tubers; that by certain
plants layers are sent out, take root, and develop new individuals; and
that many plants can be reproduced from cuttings. Further, in the mould
that quickly covers stale food, and the infusoria that soon swarm in
water exposed to air and light, he sees a mode of generation which,
seeming inexplicable, he is apt to consider "spontaneous." The reader of
popular science thinks the modes of reproduction still more various. He
learns that whole tribes of creatures multiply by gemmation--by a
development from the body of the parent of buds which, after unfolding
into the parental form, separate and lead independent lives. Concerning
microscopic forms of both animal and vegetal life, he reads that the
ordinary mode of multiplication is by spontaneous fission--a splitting
up of the original individual into two or more individuals, which by and
by severally repeat the process. Still more remarkable are the cases in
which, as in the