ed a simple and satisfying explanation of those puzzling vestigial
organs, whose existence was such a stumbling-block to the teleologists.
It enabled the biogenetic law to be substituted for the laws of
Meckel-Serres and von Baer, as being in some measure a combination and
interpretation of both.

Where the concept of evolution proved itself particularly useful was in
the interpretation of structures which were not immediately conditioned
by adaptation to present requirements, such as, for instance, the
arrangement of gill-slits and aortic arches in the foetus of land
Vertebrates. Such "heritage characters" could only be explained on the
hypothesis that they had once had functional or adaptational meaning.
Why, for instance, should the blastopore so often appear as a long slit,
closing by concrescence, unless this had been the original method of its
formation in remote Coelenterate ancestors?

The point hardly requires elaboration, since it has become an integral
part of all our thinking on biological problems. It may be as well,
however, for the sake of continuity, to give one or two examples of the
historical interpretation of animal structures. The first may
conveniently be the phylogenetic interpretation of the contrast between
"membrane" and "cartilage" bones.

In his _Grundzuege_ of 1870, Gegenbaur made the suggestion that the
investing or membrane bones were derived phylogenetically from
integumentary ossifications, and this was worked out in detail a few
years later by O. Hertwig.[458]

Many years before, several observers--J. Mueller, Williamson, and
Steenstrup--had been struck with the resemblance existing between the
placoid scales and the teeth of Elasmobranch fishes. Hertwig followed up
this clue, and came to the conclusion not only that placoid scales and
teeth were strictly homologous, but also that all membrane bones were
derived phylogenetically from ossifications present in the skin or in
the mucous membrane of the mouth, just as cartilage bones were derived
from the cartilaginous skeletons of the primitive Vertebrates. In some
cases this manner of derivation could even be observed in ontogeny, as
Reichert had seen in the Newt, where certain bones in the roof of the
mouth are actually formed by the concrescence of little teeth, (_supra_,
p. 163). Hertwig considered that the following bones were originally
formed by coalescence of teeth--parasphenoid, vomer, palatine,
pterygoid, the tooth-bearing pa