84 



DISCOVERY 



having inborn blindness, thus producing a race with good 

 modal eyesight. Now why should not a reverse process 

 be the cause of partial or complete blindness amongst 

 fishes living in darkness ? Those fishes having inborn 

 blindness were just as capable of survival as their com- 

 rades having perfect but useless eyes ; consequently a 

 process which we might call " Natural Retention " has 

 retained the factor for blindness, and, by constant 

 crossing, the mode of the race in the direction of eye 

 perfection has been reduced. This theory will no doubt 

 be attacked upon the ground that the process would be 

 too slow, but critics in this direction must remember 

 that blindness is a common aberration among animals 

 whose eyes are so primitive in form ?.s those of fishes, 

 and that the factor for blindness would in all probability 

 act as a Mendelian dominant. Also the cave-dwelling 

 fish have been imprisoned for countless generations. 

 The theory has also a great advantage over the " use and 

 disuse " idea in that it can furnish with a satisfactory 

 reply those critics who attack the latter on the question 

 of the Chinese foot-binding practice. 



I put as a last example in support ot my theory' the 

 phenomenon of the absence of pigment from the egg-shells 

 of those birds which habitually lay their eggs in the 

 dark, while those that lay in more or less exposed places 

 usually produce eggs whose shells harmonise in some 

 measure with their surroundings. Natural Selection is 

 the obvious explanation of the latter ; but the theory I 

 have just put forward seems to be not only the most 

 probable, but the only explanation of the former. The 

 pigment, unlike an organ of work, vision, or hearing, can 

 hardly be supposed to dwindle from want of use, nor 

 can its disuse be directly appreciated by, or have any 

 mechanical effect on, the embryo bird. To say that 

 those birds which Jay pigmentless shells can consequently 

 make a better yolk and albumen and so produce stronger 

 young which tend to be selected is equally absurd. 

 Therefore the conception of " Natural Retention " 

 retaining those birds having a factor for pigmentless 

 shells and, by crossing, ultimately reducing almost to 

 zero the mode of the race in respect to egg-shell pigment, 

 is the only one possible. 



Yours, etc., 

 Gilbert S. Hartley. 

 Basford, Stoke-on-Trent. 

 December 27, 1922. 



a railroad. We are glad to hear that, none the less. 

 Discovery managed to get there ! 



CLIMATIC CONDITIONS IN THE ROCKIES OF 

 BRITISH COLUMBIA 



Our reviewer of a recent book on climates in the Novem- 

 ber issue of Discovery his stimulated a correspondent 

 from the Rockies to describe with enthusiasm his own 

 climate. Mr. John Gregg writes from Hudson's Hope, 

 B.C., where only fifty white people inhabit a circle of 

 five miles radius. The temperature there varies from 

 8g° to —50° F. ; in the coldest weather there are no 

 winds, in . the hottest no flies. He speaks of grizzly 

 bears, moose, goats and sheep for the hunter's delight ; 

 but the enthusiast on grizzly bears may be a little diffident 

 in making the journey thither, for it is 250 miles from 



ACROSS THE SAHARA ON MOTOR-CARS 



The recent successful expedition of the Citroen cars 

 across the Sahara has been an interesting and useful 

 experiment. There were nearly two thousand miles 

 of desert to cross ; wells were sixty miles apart, and in 

 the centre of the route taken lay the Tanesruft, two 

 hundred miles in length and possibly as desolate [an 

 area as any on earth. The camel has hitherto made 

 desert travel possible, and Captain Haywood, who in 

 1910 travelled the same route as that taken by the 

 expedition, has written in his book, Timbuctn and the 

 Great Sahara, of the risks and trials associated with a 

 caravan expedition on camels. Motor transit in ordinary 

 cars is unsatisfactory on account of the nature of the 

 ground travelled over ; the back axles frequently break. 

 The caterpillar tread of the cars used in this expedition 

 appears to have proved efficient in overcoming this 

 difficultv. 



The e.xpedition followed, for the first two hundred 

 miles of its way, a dried watercourse known as the Wad 

 Mya. Many a British soldier will remember these 

 romantic channels through the desert, carved and moulded 

 like a cavern in the Arabian Nights, where at long intervals 

 a rushing torrent flows when the rains fall — sometimes 

 only once in seven years. In ancient Egypt there was 

 a myth that those who died during each day collected 

 at nightfall by the gate of Tuat, or the other world, which 

 ran, parallel with Egypt, out in the desert. Throughout 

 its length there ran a river, along which there floated 

 the sun-god and the souls of the righteous to the land 

 of Osiris and the great god Ra. To be sure, we paint our 

 heavens to the pattern of our earth, and doubtless the 

 Nile-born Egyptian could not imagine a riverless eternity. 

 But perhaps his conception was helped by the memory 

 of some such desert torrent as that whose course the 

 twentieth-century Citroen adventurers travelled on their 

 way to desert-encircled Timbuctu. 



The e.xpedition left Tugurt, the terminus of the 

 railway south of Biskra, on December 18,. and reached 

 Timbuctu on January 7. Despite many difficulties on 

 the way, including a sandstorm and an attack by Arab 

 banditti, M. Haardt (Director of the Citroen Factory) 

 and his followers covered the distance at an average of 

 a hundred miles a day. 



RAILWAY ELECTRIFICATION 

 The author of the article on this subject appearing in our 

 January issue regrets that he made an error in stating the line 

 voltage of the Brighton Railway. Like all single-phase railways, 

 this voltage is considerably above anything practicable with 

 continuous current. The line is at present worked with a line 

 pressure of 6,700 volts and. when further extensions are taken 

 in hand, it will be raised to 11,000 volts. We would also take 

 the opportunity of saying that the paper on " Electric Railway 

 Contact Systems," referred to at the end of the article, is by 

 Sir Philip Dawson, and contains much interesting information 

 as to the results of working with the single-phase system on 

 that railway. 



