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ANATOMY, AND LIFE-niSTORY OF THE COXIFEKJE. 229 



perverted differentiation and which are alluded to under their 

 respective headiugs. 





To show the diversity in appearance which may be met within 

 one member — the leaf — Engelmann points out that in Pinus seven 

 different forms of leaves occur, as hereafter mentioned in detail. 

 Certain of these forms of leaf occur on the same plant at dif- 

 ferent periods, being characteristic either of youth or of tem- 

 porary and intermittent accessions or relaxations of growth. In 

 other cases a certain degree of permanence of a particular form 

 of branch or leaf is observable. Mention is made of these cases 

 under the head of ramiBcation and foliage, and especially with 

 reference to the Serpent-firs (Abies exceha, vars. vwnstrosa and 

 viminalis) and to the Eetinosporas of gardens. 



The Eetinosporas have been considered to form a distinct 

 and are so described in various works of authority. They repre- 

 sent, however, only various " stadia " in the life-history of certain 

 species of Thuya, of Chamacyparis, or of Juniperus. These plants, 

 under cultivation, continue year after year to produce one de- 

 scription of foliage only, and so may very readily be mistaken 

 for separate species. Thuya pisifera thus exists under three or 

 even four distinct conditions ; and if the clue were not afforded 

 by some "happy accident," such as the occasional presence of 

 two or more forms on the same bush, all four forms might be 

 considered as so many species, a fact not to be lost sight of in the 

 identification of fossil plants. The forms in question are, as has 

 been said, more or less persistent. Why, then, under favourable 

 conditions should they not remain completely so ? May we not, 

 n fact, have, as Caspary suggested in the case of the Snake-fir, 

 the beginning of a new species ? And may we not, especially in 

 such instances as the Eetinosporas, be lookers-on at the origin of 

 forms which will gradually become stable enough to constitute 

 species ? The supposition is rendered the more plausible in that 

 the variations in question are in some cases perpetuated by seed. 

 The larger proportion of seedlings from such plants doubtless 

 revert to the typical stock ; but a considerable proportion per- 

 petuate the variety. The cultivator, by the constant elimination 

 of some forms, and by the continuous protection of others, suc- 

 ceeds at length in " fixing " certain desired varieties. 



Doubtless a similar process occurs in Nature ; and in watching 

 the morphological chauges that occur in plants, we are certainly 

 witnesses, on the one hand, of the permanence characteristic 



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