This very fascinating question has occupied the attention of modern botanists to some extent, and we also have given some thought to the subject, but so far without any measure of success. On seriously following up the identity of supposed Eucalyptus hybrids, in several cases, the contention that these were the offspring of certain indicated parents, has been found to be without solid support.

Natural hybridisation in the Eucalypts, perhaps, appeals to one as a feasible explanation for the establishment of certain species which show affinities on both sides. The organs of reproduction in Eucalyptus, however, are protected by an operculum, and in many cases pollen grains are found adhering to the stigma before the operculum falls off, so that it may be just as readily supposed that the cap in the bud stage is a protection against hybridisation.

Baron von Mueller at one time did not regard hybridisation as impossible, but thought that all ordinary chances are against it, for he states : "Hybridisation does not seem to explain the origin of these aberrant forms in a genus, where cross-fertilisation is guarded by a calycine lid." (Eucalyptographia, 8th Dec, under E. cordata.)

It may be now shown that most of these supposed aberrant forms are really distinct species, and in our opinion cross-fertilisation in the Eucalypts under natural conditions is quite exceptional, especially when we know that numerous species are growing intermixed, often flowering at the same time, and so under supposed favourable conditions for hybridisation, yet preserving throughout extensive areas their specific characters with remarkable constancy.

We were very much impressed with this aspect of the question on the completion of our work on the Eucalypts of Tasmania, and it was striking to find such a large percentage of the species (quite half) of those now growing in Tasmania, which are identical, both botanically and chemically, with the same Eucalypts growing on the mainland of Australia. It is thus evident that these species were well established in both Australia and Tasmania before the latter was separated from the continent, and that in both localities they have gone on reproducing their offspring in absolute constancy ever since. It is thought that it was not earlier than tertiary times when Bass Strait was formed, and although this period is perhaps not far back geologically, yet, considered botanically, it must have been a very long time ago; but, through all this period of time only a very few species of Eucalypts have become endemic in Tasmania. It may be mentioned that since our work on the Tasmanian Eucalypts was completed, E. acervuia, which was considered as endemic in Tasmania, has also been found growing in South Australia; and possibly when the Eucalypts on the Australian Alps shall be more completely studied, other Tasmanian species will be found to exist on the mainland also. How very different this condition of affairs must have been if hybridisation were a common occurrence with the Eucalypts. It is thus evident that very definite knowledge is needed before one can accept the statement that such and such a Eucalyptus tree is a hybrid under natural conditions.