t which he probably owed to the German
transcendentalists (see Chap. VII.), that the permanent structure of the
lower animals could be compared with phases in the development of the
higher, and particularly of man, or, as he put it, that comparative
anatomy was often only a fixed and permanent anthropogeny, and
anthropogeny a fugitive and transitory comparative anatomy (xi., p.
106).

"In rising towards the first formations," he writes, "transcendental
anatomy recognised that one and the same organ, however complicated its
definitive form might be, repeated in its transitory states the organic
simplicities of the lower classes. Thus the primitive heart of birds was
first of all a canal, then a pocket or single cavity, then finally the
complex organ of the class. Comparative anatomy was thus seen to be
repeated and reproduced by embryogeny" (xii., p. 85).

His explanation of the fact of repetition is that, "in animals belonging
to the lower classes the _formative force_, whatever it may be, has a
less energetic impulsion than in the higher animals, and hence the
organs pass through only a part of the transformations which those of
the higher forms undergo; and it is for this reason that they show
permanently the organic dispositions which are only transitory in the
embryo of man and the higher Vertebrates. Hence these double aortas,
these double venae cavae which one observes more or less constantly among
reptiles" (xxi., p. 48).

The number of stages in embryogeny is proportionate to the complexity of
the adult; the younger the embryo the simpler its organs--such is the
general formula of the relation between the embryo and the adult. But
here in Serres' doctrine of parallelism a complication enters. He
observed that embryonic organs did not always develop in a piece, by
simple growth, but often were formed by the union of separately formed
parts or layers. Thus the kidney in man is formed by the fusion of a
number of "little kidneys," and the spinal cord reaches its full
development by the laying down of successive layers within it. He was
greatly impressed with this fact, which, as a convinced believer in
epigenesis, he used with great effect against the preformistic theories.
"This method of isolated formation," he wrote, "is noticed in early
stages in the thyroid, the liver, the heart, the aorta, the intestinal
canal, the womb, the prostate, the clitoris, and the penis" (xi., p.
69). So, too, in the development o