184 THE STRUCTURE OF EVOLUTIONARY THEORY
ferent environments—a pond and the body of a complex creature—encourage
disparate inceptions for sequences of progress, then the two forces commingle
(Fig. 3- 2 ).
(2) Ramification at the top. Lamarck could not rank the vertebrates in linear
order. He followed the conventional path of fish to reptile, but could not convince
himself that birds fell between reptiles and mammals in a genealogical sense. He
therefore permitted a fork, provoked by the environmental set of forces, at the very
top of a ladder supposedly built by the unilinear impetus of progress (Fig. 3-3):
"We cannot doubt," he wrote with characteristic certainty (1809, p. 176), "that the
reptiles by means of two distinct branches,
3 - 2. Lamarck's later conception of two chains of being with different starting points, the first (to
the left) from free-living single-celled infusorians, the second (to the right) beginning with
parasitic worms spontaneously generated within the bodies of higher organisms. From Lamarck,- (Author's collection.)