2 5 8 



SCIENCE-GOSSIP. 



von Linden ( M ). She comes to the conclusion that 

 sculpturing and markings on shells are not useful, 

 and hence have not arisen by natural selection ; 

 but that they are the inherited effect of external 

 conditions, this being, as Eimer says, an efficient 

 cause. The similarity between shell -colour and algae 

 of similar habitat is the result of like physico- 

 chemical conditions, and she very rightly points 

 out that during life, algae, etc., often quite conceal 

 the surface of the shell, not only in mollusca, 

 but also in Crustacea, etc. Similar views were 

 expressed by C. Darwin ( 84 ). Talking of the 

 beautiful colours of many shells, he says : " The 



colours do not appear in most cases to be of any 

 use as a. protection ; they are probably the direct 

 result, as in the lowest classes, of the nature of 

 the tissues, the patterns and the sculpture of 

 the shell depending on its manner of growth." 

 Whichever general view we take of the colours — 

 whether useful or useless — the fact remains that 

 marine mollusca are as a rule possessed of a more 

 or less elaborate system of coloration or orna- 

 mentation, or both ; I am inclined to agree with 

 those who attribute this to the action of natural 

 selection. 



{To be continued.) 



THE PHYSICAL STATE OF MARS. 

 By Colonel H. S. Knight, F.R.A.S. 



T HAVE read Mr. Oswald's paper in Science- 

 Gossip of August, 1897, up° n " The Physical 

 State of Mars," and though the article is written 

 with so much ability, there are some statements 

 to which I would take exception as not agreeing 

 with my own observations. In the first place, the 

 writer mentions that the atmosphere of Mars is 

 one of extreme tenuity, and he treats this as an 

 explanation for the absence of clouds, except to a 

 very slight extent. I enclose a small sketch of 

 Mars as I saw it upon one occasion, which proves 

 that this planet may be very cloudy at times. I 

 think it also settles the question 

 of absence of water from the 

 greater part of its surface. 

 Again, it is a well - known 

 fact that its Polar ice - cap 

 has by no means the per- 

 manence it holds on our 

 earth, although its seasons 

 are of much longer dura- 

 tion. Now the force de- 

 veloped by any matter depends 

 chiefly on its mass. Thus, 

 when we find heat developed 

 on the surface of Mars, it is 

 a proof that its atmosphere Clouds seen on 



must be extensive. When 



the reverse is the case on our earth, near or even 

 within the tropics, the snows on high moun- 

 tains last through all time, although seldom 

 renewed, because on them the air is rare. Mars 

 being much more distant from the sun than the 

 earth, therefore, should have a colder climate. 

 With an equally dense atmosphere in Great Britain, 

 we are enveloped in mists, producing an obscura- 

 tion in our atmosphere which does not exist in dry 



t 33 ) Zeitschr. f. wiss. Zool , lxi. (1896), p. 261 ; abstract in 

 Journ. Roy. Microsc. Soc, 1S96, p. 303. 

 (3») " Descent" (1890), p. 263. 



continents such as in the Indian section of Asia, 

 especially in its north - west portion, where the 

 atmosphere is incomparably clearer. 



With regard to the remark that there are no 

 mountain ranges in Mars, we must bear in mind 

 its great distance from the earth. The altitude of 

 our highest mountains is comparatively trivial, and 

 only reached after many miles of rise from sea- 

 level, and, as observed from Mars, would appear 

 flat land. Someone observing Mars with a good 

 telescope on the Andes not only saw mountains, 

 but observed them covered with clouds which 

 deposited snow on them, and 

 this snow remained several 

 days in view, and then gradu- 

 ally disappeared. Where there 

 is snow, there must be sufficient 

 cloud formation to produce 

 rain, and also much water on 

 the surface of Mars to produce 

 clouds, or even cause a deposit 

 of frost. Thus in the dry 

 climate of Ladak frost is un- 

 known, although the winter 

 temperature falls much below 

 freezing, and this place is only 

 about il.ooo feet above sea- 

 level. 

 I have never observed " canals " on Mars, 

 although I have seen its ice-cap and dark border 

 very distinctly. I have seen reddish and buff-tinted 

 clouds, which gave me the idea of Indian and 

 North African dust storms. When the slight 

 nature of the Polar snow-cap is considered, it is 

 difficult to conceive that canals could convey water 

 for thousands of miles towards its equatorial region, 

 and in such a dry climate, even if we leave out the 

 machinery necessary to force the water over such 

 distances, for it is clear it would not flow otherwise. 

 The Observatory, Harestock, Hunts. 



the Planet Mars. 



