Dec. 12, 1889] 



NATURE 



^3^1 



first letter, we are at liberty to impgine a time when there was 

 much more land than there is at present, and when all the 

 oceans were comparatively shallow. A. J. Jukes-Browne. 



Galls. 



Before rushing into arguments on this subject, it appears to 

 me that more good might be done by entering into investiga- 

 tions of the physiological and morphological problems involved. 



A gall- fly of a particular species inserts an egg in a certain 

 position on a certain plant (oak, for instance). Another gall-fly 

 of a different species inserts its egg almost in the same position 

 on the same plant. But the results are totally dissimilar. An 

 abnormal growth is set up, from irritation, in either case ; but 

 the nature of this growth is quite difierent. The initial irritation 

 is setup by the presence of the egg, and in most gall-insects the 

 ^gg A''''""'-*"— 'hat is to say, it increases vastly in size before the 

 larva is hatched. The irritation is continued by the larva, and 

 the gall is produced, varying in form in accordance with the 

 species of gall-fly that deposited the e^g. J5ut I want to know 

 in what consists the difi"erence in the active irritation that causes 

 so great a divergence in the results ? I am not aware that this 

 has ever been answered. But I am quite sure it could be 

 answered on purely physiological grounds if carefully studied. 

 The answer would not in the least detract from the importance 

 of the point as regards natural selection ; but it might very 

 materially modify speculative theories based on results only, 

 without a precise knowledge of the agencies that produced those 

 results. R. McLachlan. 



Lewisham, November 29. 



Although I see no need of a better explanation than Prof. 

 Romanes's (Nature, Novtmber 28, p. 80) of the difficulty 

 which galls seem at first sight to present for natural selection, 

 yet I beg leave to say some words of further elucidation. 



When it was said by Darwin (" Origin of Species," 

 chap, vi.) : •♦ If it could be proved that any part of the 

 structure of any one species had been formed for the exclu- 

 sive good of another species, it would annihilate my theory, 

 for such could not have been produced through natural selec- 

 tion," he evidently meant only species living without organic 

 connection with each other, viz. his own example of the rattle- 

 snake. The argument does by no means apply to organisms 

 living in a relation of symbiosis, as is the case with gall-bearing 

 plants and the larvae inhabiting the galls. ^ Such associations form, 

 as it were, one compound organism. Natural selection evidently 

 may act in favour of each symbiont separately, provided only 

 that the effect will not damage the other symbiont in such a 

 degree as seriously to impair its existence. Some "disin- 

 terested" expenditure of energy and of organic substance is not 

 excluded by natural selection, but may be promoted, if of 

 advantage to the other partner. Thus the production of galls 

 will scarcely do any serious injury to an oak, and even if such 

 were sometimes the case, there would be no comparison 

 to the damcge worked, for instance, by Trichinae, on the 

 organism of man and animals, which hosts, nevertheless, in 

 consequence of the stimulus caused by the parasite, afford the 

 substance for capsules protecting the worms, just as plants pro- 

 duce manifold structures beneficial to the gall-insects. If 

 Trichinae would attack a species of mammals as frequently as, 

 for instance, leaf-cutting ants attack some tropical plants, then 

 those hosts would be forced either to develop, by survival of the 

 fittest, some protection against their invasion, or they would 

 succumb to the enemy and die out. 



Analogous examples might be multiplied of both plants and 

 animals, and it is especially to be remembered, as alluded to by 

 I'rof. Romanes, that the chemical activities of parasites, includ- 

 ing the elaboration of ferments affecting the saps and tissues of 

 the host, are as much under the guidance of natural selection as 

 are their morphological variations. D, Wetterhan. 



Freiburg, Badeaia, November 30. 



With all due deference to your able correspondents Dr 

 St. George Mivart and Prof. G. J. Romanes, I canno'^ 



Parwin's thorough arquaintance with these important structures is 

 shown by his elaboraie discus.'- ion in " Animals and Plants under Doniestica- 

 """•"chap. .xxiii. (2nd ed. vol. ii. p. 272). It is particularly to be notfd 

 that Datwin insists en the accordance of galls, for instance, on rcses, with 

 structures arising through bud-variati»n. 



for the life of me understand how the theory of natural 

 selection can be seriously assailed by investigations into 

 the formation of galls by insects. Gall-formation has always 

 appeared to me to be a pathological, that is a perz-erlcd 

 physiological process, and to be due to the action of some animal 

 irritant upon normal vegetable tissues during their period of 

 active growth. These formations are therefore, to my mind, fair'y 

 on a par with the globular nests produced by the larvae of the 

 CEstrus, or bot-fly, in the hides of oxen ; or to the inflammatory 

 foci in the tissues of the kidneys, due to the translation of Bacilli, 

 in the case of ulcerative endocarditis. Other examples bearing 

 on the subject will doubtless occur to your readers. In all such 

 instances we have certain changes in the cellular or protoplasmic 

 tissue-elements of the host, brought about by the growth and 

 development of a foreigner in their midst ; and natural selection, 

 in so far as it operates in such cases, seems to have sided mostly 

 with the stranger, and to be to his advantage alone. That the 

 host under these circumstances performs actions " which, if not 

 self-sacrificing," are at least "disinterested," must be admitted ; 

 but it is the self-sacrifice of coercion and disinterestedness under 

 compulsion. W. Ai.nslie Hollis. 



Brighton, December i. 



Luminous Night Clouds. 



The many inquiries and appeals regarding observations of 

 luminous night clouds which have recently appeared in the 

 columns of Nature, and the growing importance of the subject, 

 will justify me, perhaps, in sending to you, for publication in 

 that journal, the following item, so long after the event it 

 describes took place. 



About the middle of November 1887, between eight and nine 

 in the evening, as I was walking homewards from my day's work, 

 I noticed what appeared to me to be the arch of a rainbow very 

 low above the western horizon, and of a snow-white colour. A 

 bank of clouds was rapidly approaching from the west, which, at 

 the time of the first appearance of the arch, covered nearly half 

 the sky, the eastern half being clear. The arch appeared to 

 move eastwards, with and in the midst of the clouds, for it con- 

 tinually rose above the horizon, and, in the course of about half 

 an hour, had approached the zenith. 



At this time I called rut several people to witness the 

 phenomenon, which certainly presented a most extraordinary 

 appearance. The arch appeared to be uniformly of about 

 3° or 4° in width, and extended north-north-east and south- 

 south-west across the whole sky. The latter was about wholly 

 overcast with the clouds at this time, except the arch, which 

 presented a glaring brightness, and illuminated the earth 

 with a weird splendour four tr five times exceeding that of the 

 brightest moonlight. 



While at the zenith, the stars shone through the entire width 

 of the arch with apparently more than ordinary brightness ; but 

 as the arch approached towards and receded from that point, 

 the width of the transparency was observed to diminish rapidly 

 with the distance, until at 10^ or 15° on either side the stars 

 were invisible through it. 



The phenomenon appeared to be a division in the cloud 

 stratum, the opposite walls of which were pretty clearly defined ; 

 and there appeared to be absolutely nothing between these op- 

 posite cloud walls but the purest air and the white light of the arch. 

 I remember also that the wall or border of cloud on either side 

 of the arch was slowly revolving upon an axis parallel with the 

 arch ; just as is often seen in the fiont bank of clouds of an 

 approaching storm. But I do not remember the direction of the 

 rotation, or whether both borders rotated in the same or in 

 opposite directions. 



The arch moved towards the east at about the same pace 

 that it approached from the west, and with apparently the same 

 width and direction of extension. There was no moonli^^ht at 

 the time, and only a gentle breeze was blowing. The weather 

 preceding the phenomenon was fine for several weeks ; but a 

 few days afterwards, or on November 19, there was a sudden 

 and extraordinary fall of the temperature, accompanied by some 

 snow and very high wind. 



I have thought that possibly this phenomenon might throw 

 some light on the subject of luminous clouds, and that this 

 tolerably accurate description of it may therefore be of interest 

 to the students of that subject. I may add, however, that the 

 luminosity of the arch did not appear to proceed directly from 

 the clouds themselves, but from the clear space between the 



