520 EVOLUTION OF THE EUCALYPTS, 



ceediiigs, Yol. xxxvii.] that the term primary or juvenile should 

 always be adopted for the early type of foliage, and the term 

 secondary, or adult, or mature for the later type of foliage. 

 Whatever the juvenile type may be, the leaves tend very quickly, 

 after the first pair, to assume that character. Where the 

 primary leaf-type is sessile, it is usual for the first pair to be 

 shortly petiolate. The leaves of the various species of the 

 Corvmbosa-group have a great resemblance to one another, being 

 petiolate, alternate, often peltate, and covered with glandular 

 hairs. In the Stringybark-group, the primary leaves are ovate- 

 lanceolate, shortly petiolate or almost sessile, and also hairy. 

 In the Peppermint-group, the leaves are usually sessile and 

 smooth; while in E . ylohulus and its near relatives, they are 

 sessile, smooth, and glaucous. The majority of the species with 

 small cotyledons have smooth and petiolate, primary leaves. 

 Those with Y-shaped cotyledons usually have almost linear leaves 

 at first, these tapering into a short petiole. If a branch springs 

 from the axils of the cotyledons, its leaves are always of the same 

 form as those of the main stem. A curious abnormality of the 

 leaves is often seen. It is symmetrical, and occurs in both 

 members of a pair of the early leaves, and has been observed in 

 a number of species. It consists of a rounding off and shorten- 

 ing of an otherwise subacute leaf, which terminates, instead, in 

 a short, rounded boss. It is well illustrated in the figure of E 

 melanophloia, in Plate Ixviii., fig. 7. 



Evolutionary Considerations. — Messrs. liaker& Smith, on botan- 

 ical and chemical results, have outlined a scheme of the probable 

 course of evolution of the Eucalypts; and Mr. E. C. Andrews 

 has recently given us his views on " The Development of the 

 MyrtaceceJ^ This research has demonstrated, in an unexpected 

 way, the great part in the evolution taken by the cotyledon- 

 leaves. Tristania^ Angophora, and Eucalyptus probably all had 

 a common ancestor. The two former failed to adapt themselves 

 to the varying conditions of climate and soil, while Eucalyptus 

 acquired that knack. The same type of cotyledon prevails in all 

 the Angophoras, the same- type of leaf and oil, and not being 

 able to depart therefrom, they have not been able to spread far 



