I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed. 查尔斯·达尔文.物种起源.
Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. 查尔斯·达尔文.物种起源.
The ordinary and open flowers can be intercrossed; and the benefits which certainly are derived from this process are thus secured. 查尔斯·达尔文.物种起源.