292 



SCIENCE-GOSSIP. 



tion. The circumstances are such under which 

 degeneration might be expected, and the results 

 are such as one would expect to accrue from 

 degeneration. This I understand to be the ex- 

 planation of this form of shell ; assuming, that is, 

 that the marine species from which they are derived 

 had a coloured ornamented shell. This is rendered 

 not unlikely, assuming again that they are derived 

 from species having any considerable shell at all f 21 ), 

 from a consideration of recent marine mollusca, 

 and is further supported to some extent by palaeon- 

 tological evidence f 2 ' 2 ). If, however, it is held that 

 their ancestors in the sea were uncoloured and un- 

 ornamented like themselves, their type is an archaic 

 one. This is the construction which Darwin appears 

 to put upon the facts, on the ground that the 

 absence of competition would lead to a permanence 

 of form and no great development of fresh forms ; 

 he, however, says very little about the group f 23 ). 



It has recently struck me that the early pig- 

 mentation in Limnaea stagnate looks remarkably 

 like the remains (or the beginning) of a band in 

 the region of the periphery ; but the indications 

 are not as definite as they might be, to indicate a 

 hypothetical banded ancestry. 



There are some grounds, as mentioned above, 

 for supposing that Limnaea and its allies have 

 been derived immediately from terrestrial forms. 

 In this case their ancestors may have been un- 

 coloured too, for is impossible to trace coloration 

 through the profound change from an aquatic to a 

 terrestrial habit. The Pectinibranchiate forms, such 

 as Bithinia, have almost certainly not gone through 

 any terrestrial line of descent, being derived more 

 or less directly from marine forms ; so that here, 

 at any rate, there is a case in point. W. J. 

 Sollas ( 2i ) is of opinion that this thin shell has 

 arisen by natural selection, and is correlated with 

 the lower specific gravity of fresh water ; the snails 

 are thus enabled to move about more easily. Con- 

 sidering the very complete hydrostatic apparatus 

 possessed and used by the Limnaeidae in their 

 lung, with which they can alter their specific 

 gravity very readily and accurately according to 

 their requirements, this does not seem a very 

 satisfactory explanation, though it possibly has 

 some weight, especially in the non-pulmonate 

 genera, where, however, the reduction in thickness 

 is not nearly so great. 



[To be continued.) 



NOTES ON PLANT LIFE. 

 By P. Q. Keegan, LL.D. 



Autumn Tints. 

 'HPURNING over a volume of Longfellow's 

 Poems, I found the following passage de- 

 scriptive of autumn in America : " The gentle 

 wind, a sweet and passionate wooer, Kisses the 

 blushing leaf, and stirs up life Within the solemn 

 woods of ash deep-crimsoned." 



Here we have the autumnal foliage of the ash 

 tree depicted as being coloured a deep crimson. 

 The point may seem a trivial one, but in reality it 

 is very important. The first query is, does the poet 

 here allude to the common ash (Fiaxinus excelsior) of 

 our woods and groves ? Likely not, for it is known 

 that our tree is represented in North America by a 

 closely allied species. Wherefore should its leaves 

 be deep-crimsoned in the Fall, while ours at that 

 season is only lemon-yellow, passing to a dark 

 muddy brown ? Is the former nearer relatively to 

 its northern limit of growth than the latter ? We 

 have an analogous case in the late leafage of the 

 common privet (Ligustntm vulgare) of our hedges, 

 which exhibits a dark crimson or purplish ash shade. 

 I have chemically examined this phenomenon, and 



( 2I ) Some authorities think that the Limnaea ancestor is to 

 be found in the more or less shell-less groups. 



(* 2 ) Some geologists of whom I have enquired have dis- 

 agreed with me here. It may be that it has been the custom 

 in works on the subject to figure only the more ornamented 

 and decorative species. 



I 23 ) " Descent," ed. vl. p. 83. 



can testify that the " chromogen " of this colouring 

 matter is not identical or even analogous with the 

 corresponding pigments of the oak, sycamore or 

 Norway maple leaves. Similarly, I have studied 

 the "chromogen" of the mystic ash, and have 

 found that it is identical apparently with that of 

 the privet ; but while it is not developed into a 

 visible pigment in the former case with us, in 

 America the influence of the environment has 

 effected the impressive change in the closely allied 

 species to which the poet in the foregoing passage 

 most probably alludes. 



Disseminating Yeasts. 



In the January number of this journal [ante p. 

 222) " the part played by living organisms in dis- 

 seminating the yeasts to which the fermentation of 

 fruits, etc. , is due " is considered ; and in this connec- 

 tion it is profitable to recall a paper in the " Annates 

 des Sciences Naturelles," by M. Boutroux, pub- 

 lished some ten years ago or more, wherein he 

 remarks : " The species of yeast which are so abun- 

 dant in fermenting wine arise from the germs which 

 were clinging to the grape prior to the mashing, and 

 brought there by insects, and they predominate in 

 the fruit worts (must) only by their very great 

 power of proliferation on these media." He also 



( 21 ) Sci. Trans. Roy. Dublin Soc. (2) lii. p. 103. 



