October 14, 1920J 



NATURE 



213. 



combination of genetic characters which living forms 

 exhibit ; the palaontologist shows that these varying 

 degrees of similarity and dissimilarity have been 

 brought about by progressive differentiation in both 

 time and place ; but, as Dr. Bather rightly insists, 

 succession (progressive differentiation) docs not of 

 necessity imply descent. The final step in the argu- 

 ment for evolution (i.e. the theory that progressive 

 differentiation has been effected through the agency 

 of the process of reproduction) is that organisms are 

 known to be derived only from pre-e.\isting organisms, 

 and that new genetic characters are from time to 

 time differentiated in the actual course of normal 

 generation ; hence to interpret the diversity of 

 genetic characters in living forms in terms of experi- 

 ence it can only be inferred that such diversity has 

 been brought about in the course of descent. 



It appears to me that the paramount necessity for 

 clear statement on the logical position of the evolu- 

 tion theory is : (i) To recognise that much of the 

 reasoning employed in the past originated in the 

 emotional atmosphere created by popular prejudice 

 and hostility; and (2) while appreciating the fact of 

 specificity (genetic stability), to dispense entirely with 

 the arbitrary notions connected with the term 

 "species" as employed by systematists. It is 

 interesting to- note in conclusion that Darwin himself 

 regarded the facts of ontogeny as an extension of the 

 law of unity of type rather than a contention sui 

 generis in favour of the theory of evolution. 



Lancelot T. Hogben. 



Imperial College of Science, 



South Kensington, S.VV.7, October 2. 



I AM much obliged to you for letting me see Mr. 

 Hogben 's most interesting letter, and I thank him 

 for emphasising the fact that the mutants of the 

 experimental breeder do not show such recapitulatory 

 phenomena as do the mutations of the palaeontologist. 

 Since this point was dealt with, however imperfectly, 

 in the address, I surmise that Mr. Hogben has con- 

 sidered only the extract published in Nati.re. His 

 statement of "the final step in the argument for evolu- 

 tion " appears to me consequent, but 1 am not yet 

 prepared to admit that my statement was incon- 

 sequent. Neither, I fear, is absolutely conclusive. 

 Consequent or inconsequent, I did mv best to view 

 the problem without prejudice or emotion, but I 

 plead guilty to some attempt at humour. 



F. A. Bather. 



A Fracture-surface in Igneous Rock. 



The accompanying photograph (Kig. i) was taken 

 by me some years ago during the construction of the 

 Shirawta Dam, Bombay Hydro-Electric Works, 

 India. It shows a curious fracture surface due to 

 a heavy gelignite detonation in finely crystalline 

 " trap " rock. .So far as I can rememl>er, I had seen 

 other examples of this phenomenon, but photographed 

 this as it was n particularly good one, and I thought 

 it would be of special interest to "elasticians." 



B shows the "splash effect," having its origin at 

 the bottom of the ij-in. diameter vertical drill-hole A. 

 C is a two. foot rule used to fix the scale. E is the 

 vertical edj<e of a fault (or possiblv a dvke) in the 

 "trap" rock. D points to one of the faint radial 

 " splash " lines that form a sort of aurora about the 

 explosion centre .\. 



It will he noticed that the "solash" at B look* 

 like the fluting of a large fn«sil. The Derran " trap," 

 however, is an igneou* rock, and, of course, has no 



NO. 2659, VOL. 106] 



fossils in it. Tentative explanations that may be 

 advanced are (a) that the pressure at ."V was so 

 enormous at the moment of detonation that an actual 

 flow of the rock took place ; (b) that the fluted surface 

 is the result of unequal stress distribution due to 

 " interference " between waves reflected from the 

 three reflecting surfaces. These surfaces are : 

 (i) The rock surface some 10 ft. to 20 ft. above and 

 parallel to the foot-rule C. (2) The face of the fault E. / 

 (3) The original face of the cutting lying in a plane 

 parallel to the plane of the paper and, at the most, 

 2 ft. in a line normal to the paper from the points 

 .A and B. 



The distance to the original face before the blast 

 would not be more than 6 in. to i ft. from the top 

 of the drill-hole shown. The hole was drilled in the 

 side of the rock cutting having a " batter " of about 

 one in five, at the stage when the photograph was 

 taken. The drill-hole A would probably be about 

 3i ft. deep, and the point A about 5 ft. above the 

 floor of the cutting. 



It should also be mentioned that the site of this 

 explosion was the side of a rock cutting about half 

 a mile long, with level bottom leading to the 

 Shirawta-Walwhan tunnel. The rock cut at the 

 shallow end would be about 3 ft. deep, and at the 

 tunnel end about 40 ft. ; its top width was 20 ft.. 



Fig. I. — .Splash-like fractuie-iturface tlue to Kcliiiiiile exfilcsion in rock 

 A, baM of i^'' diamctrr drill-htilc and < rigin of explosion ; B, fluted 

 "»ptash effect" ; C, a-'t. tule, to liz scale ; D, one of a number of 

 radinl " splash ' lines. 



and its width at the bottom was, at that date, from 

 S ft. to 10 ft. 



Samples of rock similar to that blown away from 

 A were weighed, and their specific gravity worked 

 out at between 2720 and 2-7.i;2 (taking the weight of a 

 cubic fcxjt of water as 62-5 lb.). 



(Prof. B. Hopkinson calculated that the maximum 

 pressure at the face of an explosion of i oz. of gun- 

 cotton is of the order 100 tons per sq. in., and that 

 this maximum is attained in i /400,000th of a second.) 



.Since writing the above, it has been suggested that 

 the phenomenon is a large example of conchoidal 

 fracture. In any case, I think the photograph will 

 be of interest to readers of Nature. 



W. Bbvan Whitney. 



Meadow House, Lavters Wav, 



Oerrards Cross, Bucks, .^pfember 19. 



A Visual Illusion. 



The visual illusion described by Mr. Turner in 

 Nature of October 7, p. 180, mav be seen very well 

 by looking steadf.tstly at a long luggage train as in, 

 for example, counting the trucks at about 100 vards 

 distance. Immediatelv after the train has passed, the 

 embankment appears to slide along in the opposite 

 direction. A. E. Boycott. 



17 Loom Lane, Radlett, October 10. 



