degree, has also been seen to exist between the auditory organs of
fishes and of Cephalopods. Remarkable similarities between certain
placental and implacental mammals, between the bird's-head processes of
Polyzoa and the pedicellariae of Echinoderms, between Ichthyosauria and
Cetacea, with very many other similar coincidences, have also been pointed
out.

Evidence has also been brought forward to show that similarity is sometimes
directly induced by very obscure conditions, at present quite {222}
inexplicable, _e.g._ by causes immediately connected with geographical
distribution; as in the loss of the tail in certain forms of Lepidoptera
and in simultaneous modifications of colour in others, and in the direct
modification of young English oysters, when transported to the shore of the
Mediterranean.

Again, it has been asserted that certain groups of organic forms seem to
have an innate tendency to remarkable developments of some particular kind,
as beauty and singularity of plumage in the group of birds of paradise.

It has also been contended that there is something to be said in favour of
sudden, as opposed to exceedingly minute and gradual, modifications, even
if the latter are not fortuitous. Cases were brought forward, in Chapter
IV., such as the bivalve just mentioned, twenty-seven kinds of American
trees simultaneously and similarly modified, also the independent
production of pony breeds, and the case of the English greyhounds in
Mexico, the offspring of which produced directly acclimated progeny.
Besides these, the case of the Normandy pigs, of _Datura tatula_, and also
of the black-shouldered peacock, have been spoken of. The teeth of the
labyrinthodon, the hand of the potto, the whalebone of whales, the wings of
birds, the climbing tendrils of some plants, &c. have also been adduced as
instances of structures, the origin and production of which are probably
due rather to considerable modifications than to minute increments.

It has also been shown that certain forms which were once supposed to be
especially transitional and intermediate (as, _e.g._, the aye-aye) are
really by no means so; while the general rule, that the progress of forms
has been "from the more general to the more special," has been shown to
present remarkable exceptions, as, _e.g._, Macrauchenia, the Glyptodon, and
the sabre-toothed tiger (Machairodus).

Next, as to specific stability, it has been seen that there may be a {223}