s of thousands of generations), from ABFI, etc., up to A;
then, as each generation would only differ from the preceding as
offspring from parents, he would be unable at any point to distinguish a
species; at most, he would observe a slightly marked variety. ABFI and
ABFJ would grow more and more alike, until they became indistinguishable
in ABF; ABF and ABE would merge into AB; AB, AC, AD and AX would merge
into A. Hence, the appearance of species is due to our taking
cross-sections of time, or comparing forms that belong to periods remote
from one another (like AX, ADG, and ADHK, or AD, ADH and ADHK), and this
appearance of species depends upon the destruction of ancestral
intermediate forms.

(3) The hypothesis of development modifies the logical character of
classification: it no longer consists in a direct induction of
co-inherent characters, but is largely a deduction of these from the
characters of earlier forms, together with the conditions of variation;
in other words, the definition of a species must, with the progress of
science, cease to be a mere empirical law of co-inherence and become a
derivative law of Causation. But this was already implied in the
position that causation is the fundamental principle of the explanation
of concrete things; and accordingly, the derivative character of species
or kinds extends beyond organic nature.

Sec. 9. The classification of inorganic bodies also depends on causation.
There is the physical classification into Solids, Liquids, and Gases.
But these states of matter are dependent on temperature; at different
temperatures, the same body may exist in all three states. They cannot
therefore be defined as solid, liquid, or gaseous absolutely, but only
within certain degrees of temperature, and therefore as dependent upon
causation. Similarly, the geological classification of rocks, according
to relative antiquity (primary, secondary, tertiary, with their
subdivisions), and mode of formation (igneous and aqueous), rests upon
causation; and so does the chemical classification of compound bodies
according to the elements that enter into them in definite proportions.
Hence, only the classification of the elements themselves (amongst
concrete things), at present, depends largely upon empirical
Coinherence. If the elements remain irresolvable into anything simpler,
the definitions of the co-inherent characters that distinguish them must
be reckoned amongst the ultimate Uniformitie