Hybridisation. 



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 Kucalypts, 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 : ' Hybridisa- 

 tion 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. cor data.} 



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. 



\Ye 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 

 K 'producing 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. acervula, 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. 



