xxviii PROOFS, ILLUSTRATIONS, AUTHORITIES, ETC. 



the shock. If the physical revolution supposed to be going forward 

 is arrested or recedes, the individuals which had not passed the cul- 

 minating point remain as a fifth variety, or relapse towards their 

 former station ; whilst the few which have crossed the barrier remain 

 permanently behind it, even under the partial retrogression of the 

 causes to which they owed their newly-developed organization. We 

 may suppose some species and individuals to be more able to pass 

 than others, and that many become extinct from inability to accom- 

 plish it. Under this point of view, a hiatus, rather than a regular 

 passage, is regarded between a species, and that whence it is sup- 

 posed to be derived, just as two crystals may occur, nearly identical 

 in composition, but without an insensible gradation of intermediate 

 forms, the laws, both of organic and inorganic matter, requiring 

 something definite : whence the rarity of hybrids and monsters, 

 themselves subject to established laws." Professor Hctldemans 

 Enumeration of Recent Freshwater Mollusca, Boston Journal of 

 Natural History, January, 1844. 



Barton, in his Lecture on the Geography of Plants (1827), gives 

 the following remarks from the Flora Lapponica of Wahlenberg: 

 " It is not easy to decide in every case whether the specific differences 

 of plants are invariable or not ; and I do not blush to avow, that my 

 observations in regions varying as to soil and climate, have led me to 

 form an opinion differing essentially from that of the greater number 

 of botanists. A naturalist, who has visited only level countries, such 

 as Northern Germany, can scarcely imagine how variable are the 

 appearances of plants in mountainous districts. He frames for 

 himself certain specific chai'acters, and, by their help, supposes that 

 he can determine between different species in any part of the world; 

 and, in order completely to satisfy himself on the subject, he raises a 

 few specimens in his garden. But I do not hesitate to affirm that 

 conclusions so drawn are not fit to be put in competition with the 

 extraordinary transitions observable in many parts of Lapland. Who 

 has ever been able to bring back to their original form the monstrous 

 productions of our gardens ? When, therefore, I assert that the 

 Woolly-leaved willow \_Salix lanata] is sometimes found with leaves 

 perfectly entire sometimes serrated, sometimes round, sometimes 

 spear-shaped, sometimes downy, sometimes perfectly smooth I 

 merely state the result of my own unquestionable experience, which 

 I cannot consent to overlook, for the sake of pleasing the fancies of 

 botanists." 



A correspondent adds a number of facts and remarks from high 

 authorities favourable to the hypothesis of variability of plants. 



" ' Algas,' says M. Fries, the Swedish botanist, (cent. vii. 669,) 

 ' which are much more extended in their native element, when 

 exposed to the air, contract, and become lichens.' Thus nostoc 

 muscorum becomes collena limosum ; and Sir J. E. Smith has even 

 decided that Lichen pygm&a, when growing under water, is an alga, 

 but when above, a lichen. (Loudons Encyc. of Plants, p. 978.) 

 This plant (facus pygmaus of Eng. bot.) generally grows on rocks 



