ver of lower Vertebrates and of Invertebrates. In
general he shows himself, in his comparisons, lacking in morphological
insight.

His treatment of the vascular system affords perhaps the best example of
his method (pp. 8-25). The simplest form of heart is the simple tubular
organ in insects, and it is under this form that the heart first appears
in the developing chick. The bent form of the embryonic heart recalls
the heart of spiders; it lies at first free, as in the mollusc _Anomia_.
The heart consists at first of one chamber only, recalling the
one-chambered heart of Crustacea. A little later three chambers are
developed, the auricle, ventricle, and aortic bulb; at this stage there
is a resemblance to the heart of fish and amphibia. At the end of the
fourth day the auricle becomes divided into two, affording a parallel
with the adult heart of many reptiles.

In his large text-book of a somewhat later date, the _System der
vergleichenden Anatomie_ (i., 1821), he works out the idea again and
gives to it a much wider theoretic sweep, hinting that the development
of the individual is a repetition of the evolutionary history of the
race. Meckel was a timid believer in evolution. He thought it quite
possible that much of the variety of animal form was due to a process of
evolution caused by forces inherent in the organism. "The
transformations," he writes, "which have determined the most remarkable
changes in the number and development of the instruments of organisation
are incontestably much more the consequence of the tendency, inherent in
organic matter, which leads it insensibly to rise to higher states of
organisation, passing through a series of intermediate states."[150]

His final enunciation of the law of parallelism in this same volume
shows that he considered the development of the individual to be due to
the same forces that rule evolution. "The development of the individual
organism obeys the same laws as the development of the whole animal
series; that is to say, the higher animal, in its gradual evolution,
essentially passes through the permanent organic stages which lie below
it; a circumstance which allows us to assume a close analogy between the
differences which exist between the diverse stages of development, and
between each of the animal classes" (p. 514).

He was not, of course, able fully to prove his contention that the lower
animals are the embryos of the higher, and we gather from the followi