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2. From the Chain of Being to the Tree of Life.
As should be clear from the foregoing, the frequent
references to the concatenation of creatures in
eighteenth-century science (with, perhaps, an excep-
tion for Robinet) appeal rather to the principle of
continuity than to that of plenitude. Now although
continuity is an idea implicit in that of plenitude, the
converse is not true. That nature makes no jumps—and
no scientist in the second half of the eighteenth century
appeared to doubt it—does not in fact mean that
nature realizes all those possibilities implicit from all
eternity in the act of creation. The world's continuity,
in other words, is due to the action of laws and forces
and not to a necessity inherent in the divine nature.
And with that the very foundation of the principle of
plenitude, namely, its connection with the principle of
sufficient reason stressed by Leibniz, is broken down.

Typical in this sense is, once again, Buffon's position,
when he places the principle of sufficient reason
among moral entities (êtres moraux), created by man
on the basis of arbitrary relations which can produce
“rien... de physique et de réel,” and can never be-
come a “physical reason” for things. It is therefore
illegitimate to ask ourselves the “why” of nature (His-
toire des animaux, Ch. V). Science no sooner rejects
the principle of sufficient reason than it does the prin-
ciple of plenitude, that is, the very foundation of the
Chain of Being. The principle of continuity which, as
we have seen, survives the process of temporalization,
is by itself not enough to guarantee the survival of the
Chain of Being. And the best epitaph, at the conclusion
of this sketch, might be one from the same Buffon:
le vivant et l'animé, au lieu d'être un degré méta-
physique des êtres, est une propriété physique de la
matière (“Animated life, instead of being a metaphysi-
cal grade of being, is only a physical property of mat-
ter,” ibid., Ch. I).

The succession of living forms appears explainable
now on the basis of the working of physical laws or
of an activity inherent in nature: nature, as La Mettrie
says, is “neither Chance, nor God” (ni Hasard, ni Dieu).
The certainty of this descending process, from God to
the natural world, fails; and with that is lost too the
presumption that science can reconstruct the plan of
creation in all its fullness by working its way back per
vestigias.

Now we may say that the divorce between the two
ideas, that of continuity and that of plenitude, is com-
plete, even though they appear associated once more
in romantic philosophy: in Schiller's dialectic of
Formtrieb and Stofftrieb; in Fichte's conception of the
Ego as infinite activity; in the philosophy of nature
of Schelling; in the ethico-political ideal of the reach-
ing of moral perfection through an indefinite progress;
and in the conception of aesthetic progress as infinite
productivity.

In science, the metaphor of the Chain of Being was
to continue to circulate long after its “crisis”; the
English paleontologist James Parkinson, still in the
second decade of the nineteenth century, saw in it an
obstacle to the correct interpretation of the data of

the new science (Greene [1959], p. 122). But in general,
in nineteenth-century science, another metaphor gains
currency, one that keeps the quality of continuity but
not of plentitude. It is the tree of evolution, published
by Lamarck in the Philosophie zoologique ([1809]; re-
produced in Greene, p. 163), which represents a series
branching off in an irregularly spaced but uninter-
rupted way. It is what Darwin calls the “Tree of Life”
(Origin of Species [1859], Ch. IV).