between
offspring and parents; and the appearance of Kinds existing in nature,
which is so striking in a museum or zoological garden, would entirely
vanish.

A classification, then, as formerly observed, represents a cross-section
of nature as developing in time: could we begin at the beginning and
follow this development down the course of time, we should find no
classes, but an ever-moving, changing, spreading, branching continuum.
It may be represented thus: Suppose an animal (or plant) A, extending
over a certain geographical area, subject to different influences and
conditions of climate, food, hill and plain, wood and prairie, enemies
and rivals, and undergoing modifications here and there in adaptation to
the varying conditions of life: then varieties appear. These varieties,
diverging more and more, become distinct species (AB, AC, AD, AX). Some
of these species, the more widely diffused, again produce varieties;
which, in turn become species (ABE, ABF, ADG, ADH). From these, again,
ABE, ABFI, ABFJ, AC, ADHK, ADHL, ADHM, the extant species, descend.

A
/|\\_
/ | \ \__
/ | \ \_____
________/ | \__ \
/ | \ \
AB AC AD AX
/\ | /\
/ \ | / \
/ \ | / \
ABE ABF | ADG ADH
/ / \ | /|\
/ / \ | / | \
/ / \ | / | \
ABE ABFI ABFJ AC ADHK ADHL ADHM

If in this age a classifier appears, he finds seven living species,
which can be grouped into four genera (ABE, ABF, AC, ADH), and these
again into three Families (AB, AC, AD), all forming one Order. But the
animals which were their ancestors are all extinct. If the fossils of
any of them--say AB, ADG and AX--can be found, he has three more
species, one more genus (ADG), and one more family (AX). For AC, which
has persisted unchanged, and AX, which has become extinct, are both of
them Families, each represented by only one species. It seems necessary
to treat such ancient types as species on a level with extant forms; but
the naturalist draws our attention to their archaic characteristics, and
tries to explain their places in the order of evolution and their
relationships.

But now suppose that he could find a fossil specimen of every generation
(hundred