es
animales."[100] Here the relation of his evolution-theory to his
morphology is pointed out. The principle of unity of plan and
composition cannot be the final goal of zoology; there must follow on it
a philosophical study of the _differences_ between organic forms. The
causes of these differences are to be found in the environment (pp.
66-7). Geoffroy seems here to be moving from a pure to a causal
morphology. It is probable, he continues, that living species have
descended by uninterrupted generation from the antediluvian species (p.
74), and that they have in the process become modified through external
influences.

Now of all functions respiration is the most important, and upon
respiration everything is regulated. "If it be admitted that the slow
progression of the centuries has brought in its train successive changes
in the proportion of the different elements of the atmosphere, it
follows as a rigorously necessary consequence that the organisation has
been proportionately influenced by them" (p. 76). The respiratory milieu
changes, the species change with it, or are eliminated (p. 79). We may
see, perhaps, in the stress which Geoffroy lays upon respiration and the
respiratory milieu a result of his constant obsession with the
comparison of fish with air-breathing Vertebrates.

In the first geological period, we read in another Memoir of the same
year,[101] when ammonites and _Gryphaea_ flourished, hot-blooded animals
with lungs could not exist. "A lung constructed like that of mammals and
birds would not have been adapted to the essence of the respiratory
element such as in my conception of it the system of the environing air
used to be"[102] (p. 58).

Geoffroy does not tell us exactly how the milieu is to act upon the
organism; the whole theory is little more than a sketch and a pointing
out of the way for future research--and in this prophetic enough. The
action of external agents was apparently considered as physical, and no
power of active adaptation was ascribed to the organism.

From a passage in the memoir "Sur la Vertebre" we may perhaps infer that
he believed increasing complexity of structure to be due to a
realisation of potentialities, to the development of parts present in
the lower animals only in potency--"the organisation ... only awaits
favourable conditions to rise, by addition of parts, from the simplicity
of the first formations to the complication of the creatures at the head
of the sc