The following cases are worth giving, as they relate to monoecious forms, which do not require, and consequently have not been injured by, castration. Girou de Buzareingues crossed what he designates three varieties of gourd,[232] and asserts that their mutual fertilisation is less easy in proportion to the difference which they present. I am aware how imperfectly the forms in this group were until recently known; but Sageret,[233] who ranked them according to their mutual fertility, considers the three forms above alluded to as varieties, as does a far higher authority, namely, M. Naudin.[234] Sageret[235] has observed that certain melons have a greater tendency, whatever the cause may be, to keep true than others; and M. Naudin, who has had such immense experience in this group, informs me that he believes that certain varieties intercross more readily than others of the same species; but he has not proved the truth of this conclusion; the frequent abortion of the pollen near Paris being one great difficulty. Nevertheless, he has grown close together, during seven years, certain forms of Citrullus, which, as they could be artificially crossed with perfect facility and produced fertile offspring, are ranked as varieties; but these forms when not artificially crossed kept true. Many other varieties, on the other hand, in the same group cross with such facility, as M. Naudin repeatedly insists, that without being grown far apart they cannot be kept in the least true. Another case, though somewhat different, may be here given, as it is highly remarkable, and is established on excellent evidence. Koelreuter minutely describes five varieties of the common tobacco,[236] which were reciprocally crossed, and the offspring were intermediate in character and as fertile as their parents: from this fact Koelreuter inferred that they are really varieties; and no one, as far as I can discover, seems to have doubted that such is the case. He also crossed reciprocally these five varieties with _N. glutinosa_, and they yielded very sterile hybrids; but those raised from the _var. perennis_, whether used as the father or mother plant, were not so sterile as the hybrids from the four other varieties.[237] So that the sexual {109} capacity of this one variety has certainly been in some degree modified, so as to approach in nature that of _N. glutinosa_.[238]