IV. EIGHTEENTH-CENTURY EVOLUTIONISM
During the eighteenth century evolutionary ideas in
the biological sciences gradually matured, despite
strong opposition. The history of what happened is
complex, but broadly speaking, there were changes in
basic theoretical or philosophical principles, and new
empirical discoveries made in those sciences. On the
theoretical side, they were influenced by the doctrine
of continuity which had a considerable vogue at the
time. They were also influenced by the nominalism
which had become a feature of contemporary philoso-
phy. These doctrines encouraged biologists to question
Platonistic conceptions of species, and to investigate
the anatomy, embryonic development and variability
of individual organisms. Likewise, the theoretical
model of nature as a mechanical system governed by
external laws was confronted with a rival model, due
in part to Leibniz, of nature as a self-organizing system
functioning in accordance with inner dynamic forces.
On the empirical side, the biological sciences brought
forward new interpretations of observed facts in com-
parative anatomy, embryology, and genetics which
stimulated maturing evolutionism. Three influential
figures whose work embodies these ideas were Buffon,
Maupertuis, and Diderot.
Buffon's vast Histoire naturelle, in 44 volumes
(1749-1804), contains material which, as Lovejoy has
said, “both fostered and hindered the propagation of
evolutionary ideas in biology” (Lovejoy [1959], p. 111).
The contribution of Buffon's geological views has al-
ready been mentioned. In addition, he stated quite
explicitly the hypothesis of organic evolution, without
actually espousing it. He even suggested “that man and
ape have a common origin; that, in fact, all the families
among plants as well as animals, have come from a
common stock” (Buffon [1783], IV, 382). His knowledge
of anatomical homologies and of individual variations
inclined him to espouse evolutionism. Yet on the other
hand, he publicly denied that species are mutable.
They are “perduring entities, as ancient, as permanent,
as Nature herself.” In holding this view, however,
Buffon differed sharply from his contemporary, Carl
von Linnaeus, who had defined a species Platonistically
in terms of invariant characteristics. Buffon defined a
species in terms of the relation of interbreeding, so that
two animals of opposite sex belong to the same species
if their offspring are fertile, and belong to different
species if they fail to produce offspring or produce
offspring that are sterile. It has sometimes been said
that his refusal to espouse the transmutation of species
was due to his desire to avoid the hostility of the
Church. This may have been partly the case; but he
does offer arguments in support of his position drawn
from the biological knowledge of his day (cf. Lovejoy,
1959).
The importance of Maupertuis for evolutionism lies
in the fact that he not only envisaged the transmutation
and diversification of species, but also sketched an
explanation of how these processes might have come
about. His study of embryogeny impressed on him the
frequent occurrence of deviations from the norm in
individual development “Errors” arise that produce
new characteristics of organisms and are then trans-
mitted to offspring. If these characteristics enable the
organisms to adapt to the environment more success
fully than their predecessors, a new species will result.
Repeated deviations could lead to a diversification of
species such as now obtains on the earth. The whole
process might even have started “from two individuals
alone.” Furthermore, since the developmental “errors”
may be attributed to fortuitous rearrangements of the
basic hereditary particles, no design or teleology need
be postulated. This explanation appears to anticipate
in outline much later accounts of evolution which
appeal to genetic mutations and natural selection. Yet
Maupertuis' approach was more speculative than
empirical, and his ideas remained rather vague. Hence,
despite his importance, it is overstating the case to say
that “he must be ranked above all the precursors of
Darwin” (Glass [1959], p. 74; cf. Lovejoy, 1950).
Diderot was influenced by Buffon's Histoire naturelle
and by Maupertuis' Système de la nature (1751). He
recognized that the anatomical homologies mentioned
in Buffon's work supported the idea that species evolve.
He also recognized the value of Maupertuis' conjecture
that variations which occur in individual development
might, given sufficient time, lead to an immense diver-
sification of species. He shared with both men a predi-
lection for the idea of spontaneous generation, although
he did not accept Buffon's concept of “organic mole-
cules” or Maupertuis' speculation that the basic hered-
itary particles had some rudimentary form of intelli-
gence. The most he was prepared to admit was that
“sensitivity” is either an inherent property of matter
or a property which it acquires when it reaches a stage
of sufficient organization.
The distinctive feature of Diderot's transformism is
that it is part of an evolutionary metaphysics. Like
Spencer a century later, but much less systematically,
he aimed to explain how the universe had evolved from
a primitive state towards increasing complexity and
specialization. Unlike Spencer, however, he espoused
a thoroughgoing, dynamic materialism. Matter with its
inherent property of motion, and perhaps of sensitivity,
accounts for all that has come to be. The universe is
a self-organizing whole whose parts are interconnected
and ceaselessly changing. In the course of “millions
of years,” living things have undergone “an infinite
number of successive organizations and developments.”
These have brought about the existence of sensations,
thoughts, languages, laws, sciences, and arts on the
earth. Living things have “perhaps still other develop-
ments to undergo which are unknown to us.” The
process of universal change is neither purposive nor
mechanical but organic. Like the life-cycle of plants
and animals, it may well be subject to dissolution as
well as evolution.
These formulations of evolutionism in eighteenth-
century biology met resistance from within the science
itself. The chief resistance came from embryology
which was then dominated by the version of preforma-
tionism known as the “encapsulation (
emboîtement)
theory,” defended with powerful arguments by Charles
Bonnet in his
Considérations sur les corps organisés
(1762). This was one of the first works to use the term
“evolution” in a biological sense. For Bonnet, however,
“evolution” designated the process of ontogenesis in-
terpreted as the development of an individual organism
from a germ in which it, and all its potential descend-
ants, were contained. When the world was created,
all future generations of living things were “encapsu-
lated” in a set of primordial germs. Preformationism
thus implied that the boundaries between species were
permanently fixed. The counter-theory of epigenesis,
accepted by Maupertuis, Diderot, and K. F. Wolff, was
favorable to transformism because epigenesists re-
garded hereditary variations as adding characteristics
to living things in the course of their development.
But epigenesis had become linked with the notion of
spontaneous generation, and was discredited along with
that notion by the experiments of Lazaro Spallanzani
(1729-99) and others. Hence the temporary triumph
of preformationism arrested biological evolutionism
until the period of Lamarck. This situation may have
had something to do with the fact that even so eminent
a figure as kant, who was vaguely attracted to evolu-
tionistic modes of thought, rejected the idea that spe-
cies can change.
