aw. Adjoining parts
which are not homologous sometimes cohere; but this cohesion appears to
result from mere juxtaposition, and not from mutual affinity.

In the vegetable kingdom Moquin-Tandon[847] gives a long list of cases,
showing how frequently homologous parts, such as leaves, petals, stamens,
and pistils, as well as aggregates of homologous parts, such as buds,
flowers, and fruit, become blended into each other with perfect symmetry.
It is interesting to examine a compound flower of this nature, formed of
exactly double the proper number of sepals, petals, stamens, and pistils,
with each whorl of organs circular, and with no trace left of the {342}
process of fusion. The tendency in homologous parts to unite during their
early development, Moquin-Tandon considers as one of the most striking laws
governing the production of monsters. It apparently explains a multitude of
cases, both in the animal and vegetable kingdoms; it throws a clear light
on many normal structures which have evidently been formed by the union of
originally distinct parts, and it possesses, as we shall see in a future
chapter, much theoretical interest.

* * * * *

_On the Variability of Multiple and Homologous Parts._--Isidore
Geoffroy[848] insists that, when any part or organ is repeated many times
in the same animal, it is particularly liable to vary both in number and
structure. With respect to number, the proposition may, I think, be
considered as fully established; but the evidence is chiefly derived from
organic beings living under their natural conditions, with which we are not
here concerned. When the vertebrae, or teeth, or rays in the fins of
fishes, or feathers in the tails of birds, or petals, stamens, pistils, and
seeds in plants, are very numerous, the number is generally variable. The
explanation of this simple fact is by no means obvious. With respect to the
variability in structure of multiple parts, the evidence is not so
decisive; but the fact, as far as it may be trusted, probably depends on
multiple parts being of less physiological importance than single parts;
consequently their perfect standard of structure has been less rigorously
enforced by natural selection.

* * * * *

_Compensation of Growth, or Balancement._--This law, as applied to natural
species, was propounded by Goethe and Geoffroy St. Hilaire at nearly the
same time. It implies that, when mu