he
_Origin_, Ed. i. p. 247, vi. p. 368.

{71} In the _Origin_, Ed. i. p. 258, where the author speaks of
constitutional differences in this connexion, he specifies that
they are confined to the reproductive system.

{72} The sensitiveness of the reproductive system to changed
conditions is insisted on in the _Origin_, Ed. i. p. 8, vi. p. 10.

The ferret is mentioned, as being prolific in captivity, in _Var.
under Dom._, Ed. 2, ii. p. 90.

But turning to plants we find same class of facts. I do not refer to
seeds not ripening, perhaps the commonest cause, but to plants not
setting, which either is owing to some imperfection of ovule or pollen.
Lindley says sterility is the [curse] bane of all propagators,--Linnaeus
about alpine plants. American bog plants,--pollen in exactly same state
as in hybrids,--same in geraniums. Persian and Chinese{73} lilac will
not seed in Italy and England. Probably double plants and all fruits owe
their developed parts primarily <?> to sterility and extra food thus <?>
applied{74}. There is here gradation <in> sterility and then parts, like
diseases, are transmitted hereditarily. We cannot assign any cause why
the Pontic Azalea produces plenty of pollen and not American{75}, why
common lilac seeds and not Persian, we see no difference in healthiness.
We know not on what circumstances these facts depend, why ferret breeds,
and cheetah{76}, elephant and pig in India will not.

{73} Lindley's remark is quoted in the _Origin_, Ed. i. p. 9.
Linnaeus' remark is to the effect that Alpine plants tend to be
sterile under cultivation (see _Var. under Dom._, Ed. 2, ii. p.
147). In the same place the author speaks of peat-loving plants
being sterile in our gardens,--no doubt the American bog-plants
referred to above. On the following page (p. 148) the sterility of
the lilac (_Syringa persica_ and _chinensis_) is referred to.

{74} The author probably means that the increase in the petals is
due to a greater food supply being available for them owing to
sterility. See the discussion in _Var. under Dom._, Ed. 2, ii. p.
151. It must be noted that doubleness of the flower may exist
without noticeable sterility.

{75} I have not come across this case in the author's works.

{76} For the somewhat doubtful case of the cheetah (_Felis jubata_)
see _Var. under Dom._, Ed. 2, ii. p. 133. I do n