hat their
wants would have excited them to efforts, and that continued efforts
would have given rise to new organs;" or rather to the re-acquisition of
organs which, in a manner irreconcileable with the principle of the
_progressive_ system, have grown obsolete in tribes of men which have
such constant need of them.

_Recapitulation._--It follows, then, from the different facts which have
been considered in this chapter, that a short period of time is
generally sufficient to effect nearly the whole change which an
alteration of external circumstances can bring about in the habits of a
species, and that such capacity of accommodation to new circumstances is
enjoyed in very different degrees, by different species.

Certain qualities appear to be bestowed exclusively with a view to the
relations which are destined to exist between different species, and,
among others, between certain species and man; but these latter are
always so nearly connected with the original habits and propensities of
each species in a wild state, that they imply no indefinite capacity of
varying from the original type. The acquired habits derived from human
tuition are rarely transmitted to the offspring; and when this happens,
it is almost universally the case with those merely which have some
obvious connexion with the attributes of the species when in a state of
independence.

CHAPTER XXXVI.

WHETHER SPECIES HAVE A REAL EXISTENCE IN NATURE--_continued_.

Phenomena of hybrids--Hunter's opinions--Mules not strictly
intermediate between parent species--Hybrid plants--Experiments of
Kolreuter and Wiegmann--Vegetable hybrids prolific throughout
several generations--Why rare in a wild state--Decundolle on hybrid
plants--The phenomena of hybrids confirm the distinctness of
species--Theory of the gradation in the intelligence of animals as
indicated by the facial angle--Doctrine that certain organs of the
foetus in mammalia assume successively the forms of fish, reptile,
and bird--Recapitulation.

_Phenomena of hybrids._--We have yet to consider another class of
phenomena, those relating to the production of hybrids, which have been
regarded in a very different light with reference to their bearing on
the question of the permanent distinctness of species; some naturalists
considering them as affording the strongest of all proofs in favor of
the reality of species; others, on the contrary, appealing to t