245 



Selection. — This is Darwin's expression for what Herbert Spencer has termed 

 the " survival of the fittest." This selection can be aided by man, but most of the 

 variations already rioted in the genus refer to naturally-grown forms. 



Darwin's view was that of all the causes which induce variability, excess of 

 food is probably the most powerful.* 



Hybridisation is a term implying the breeding together of members of 

 different species. The word is derived from the Latin for a mongrel. In other 

 words, two different species must be concerned. The word " crossing" is sometimes 

 taken to imply "the mingling of strains within a species"; but, in view of the 

 unequal relations of varieties and species as often defined, it seems convenient to 

 take hybridisation and crossing together, at all events for our present purpose. 



I have dealt with hybridisation as regards Eucalyptus in various publications,! 

 and need not repeat the facts and inferences at this place. 



llutation or Saltation is the term applied to sudden changes of characters 

 for which no immediate cause is apparent. The phenomena were first largely 

 investigated and brought under notice by Hugo de Vries, of Amsterdam. 



The resultant plants or sports are not hybrids, and are produced as the effect 

 of various circumstances which disturb the conditions of a plant. The tendency to 

 alteration is latent in the plant, and stimuli not always clear to us are sufficient to 

 bring out these mutation-forms. 



"When we speak of the natural or innate tendency of a plant to differ from 

 the remainder of the plants of similar origin,! we often refer to mutation-forms. 



Variation in "plants induced by environment. 



Let me quote some references by eminent botanists to this subject : — 



1. 'Pseudo-species of Botanists. — Dr. D. Mariano de la Paz Graells . . . 

 adds the following remarks upon some of the many so-called species, which he 

 shows are only modifications due to environment. Thus, of Pyrethrum sulphureurn. 

 and Dianthus brachyanthus, he writes : — 



The polymorphism which these plants acquire at different elevations has given rise to the formation 

 of distinct species, i.e., admitted as such by botanists of note. Studying the original division of P. pulver- 

 ulentum, of Lagasca, and of P. sulphureurn, of Boissier and Renter, Willkom has united them into one 

 single species, which he has called in his Prodromus Flora Hispanica, P. Hispanicum. la this, he 

 recognises two well-defined groups, the "pinnatifid" and "laciniate" types, placing in the first group 

 P. pv.lveridentum of Lagasca, and the P. radicansoi Cavanilles ; and in the second, P. sulphureurn, Boiss. 

 et Rent., which Asio had named Chrysanthemum Aragonense, and C. Bocconi or P. Bocconi, Wal. 

 P. versicolor, Willkom ; which turns out to be the P. sidphureuni, var. P. alpinum, Boiss. et Rent. 



* See "Of the Causes of Variability,'' chapter xxii of Darwin's Animals and Plants under Domestication, a most 

 interesting and valuable chapter. 



t e.g., "On Hybridisation in the genus Eucalyptus," report vol. x, Aust. Assoc. Adv. Science (Dunedin meeting, 

 January, 1904). This paper contains several bibliographical references. 



This work, e.g., V, 140 ; VI, 1G4 ; Forest Flora of New South Wales ; Proc. Linneau Soc. N.S. IF., especially 1905, 

 p. 492; Victorian Naturalist, xxi, 114, 116. 



See also Maiden and Cambage, Proc. Linn. Soc. N.S. W., 1905, p. 109. 



£ Compare my Dunedin paper (already cited), p. 297 ; also my paper, " The Variability of Eucalyptus under 

 Cultivation," Proc. Linn. Soc. N.S. W., 1904, p. 887. 



