If we now turn to the more immediate cause of self-sterility, we clearly see that in most cases it is determined by the conditions to which the plants have been subjected.  Thus Eschscholtzia is completely self-sterile in the hot climate of Brazil, but is perfectly fertile there with the pollen of any other individual.  The offspring of Brazilian plants became in England in a single generation partially self-fertile, and still more so in the second generation.  Conversely, the offspring of English plants, after growing for two seasons in Brazil, became in the first generation quite self-sterile.  Again, Abutilon darwinii, which is self-sterile in its native home of Brazil, became moderately self-fertile in a single generation in an English hothouse.  Some other plants are self-sterile during the early part of the year, and later in the season become self-fertile.  Passiflora alata lost its self-sterility when grafted on another species.  With Reseda, however, in which some individuals of the same parentage are self-sterile and others are self-fertile, we are forced in our ignorance to speak of the cause as due to spontaneous variability; but we should remember that the progenitors of these plants, either on the male or female side, may have been exposed to somewhat different conditions.  The power of the environment thus to affect so readily and in so peculiar a manner the reproductive organs, is a fact which has many important bearings; and I have therefore thought the foregoing details worth giving.  For instance, the sterility of many animals and plants under changed conditions of life, such as confinement, evidently comes within the same general principle of the sexual system being easily affected by the environment.  It has already been proved, that a cross between plants which have been self-fertilised or intercrossed during several generations, having been kept all the time under closely similar conditions, does not benefit the offspring; and on the other hand, that a cross between plants that have been subjected to different conditions benefits the offspring to an extraordinary degree.  We may therefore conclude that some degree of differentiation in the sexual system is necessary for the full fertility of the parent-plants and for the full vigour of their offspring.  It seems also probable that with those plants which are capable of complete self-fertilisation, the male and female elements and organs already differ to an extent sufficient to excite their mutual interaction; but that when such plants are taken to another country, and become in consequence self-sterile, their sexual elements and organs are so acted on as to be rendered too uniform for such interaction, like those of a self-fertilised plant long cultivated under the same conditions.  Conversely, we may further infer that plants which are self-sterile in their native country, but become self-fertile under changed conditions, have their sexual elements so acted on, that they become sufficiently differentiated for mutual interaction.