NA TURE 



{Nov. 6, 1884 



different effects of iridectomy in cases of acute and chronic 

 glaucoma. Dr. Johnson then proceeds to describe an 

 operation which he terms scleral paracentesis, and de- 

 scribes as new, but which we have seen performed both 

 by Mr. Hancock and by Mr. Power many years ago. In 

 point of fact, Mr. Hancock's operation was a scleral para- 

 centesis, and his view, which is not altogether incorrect, 

 and was based on observation, was that in glaucoma a 

 circumcorneal depression could be seen which he imagined 

 to be due to the ciliary muscle, and his section, made with 

 the same instrument recommended by Dr. Johnson, 

 namely, a Wenzel's double-edged knife, was made through 

 the sclera with the object of dividing the ciliary muscle ; 

 and the excellent results obtained in some cases show 

 clearly that the escape of the vitreous which followed the 

 incision, accompanied, when the anterior chamber was 

 opened, by the aqueous humour, was quite enough to afford 

 relief to all the symptoms and to restore vision, even if the 

 spasm of the ciliary muscle was quite imaginary. We do 

 not, however, wish to deprive Dr. Johnson of the credit of 

 having thought out this method of procedure, though he 

 may rest assured that he will meet with many cases of 

 chronic glaucoma that will derive no benefit from scleral 

 paracentesis, and that he will have to be careful in 

 promising success from his operation in such cases. 



LETTERS TO THE EDITOR 



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An Unnoticed Factor in Evolution 

 Two observed biological facts seem to oppose great difficulties 

 to any explanation on evolution principles ; difficulties admitted 

 by evolutionists as well as their opponents. I mean — 



(1) The fact that varieties produced by artificial selection, 

 however divergent, are always fertile among themselves, while 

 species supposed to have been produced naturally by an analogous 

 process are often not mutually fertile even when very slightly 

 divergent ; and 



(2) The fact that species evidently derived from a common 

 ancestor, and differing only in small points of marking, though 

 not fertile with one another, are often found side by side in 

 places where it would seem that cross-breeding must prevent 

 any division of the ancestral species into divergent branches. 



The first seems to require that a period much greater than 

 that of artificial selection should be necessary to produce sterility 

 between descendants from the same ancestor ; a supposition 

 which would require an almost incredible period for evolution as 

 a whole. The second seems to require that many species now 

 intermixed should once have been geographically separated, 

 sometimes in cases where this is very difficult to imagine. Both 

 these difficulties are completely removed if we suppose mutual 

 sterility to be not the result but the cause of divergence. 



As far as can be judged, "sports " are as likely to occur in the 

 generative elements (ova and spermatozoa) as in other parts of 

 the body, and from their similarity in widely unlike groups it 

 seems certain that a very slight variation in these elements woidd 

 render their owner infertile with the rest of its species. Such a 

 variation occurring in a small group (say the offspring of one 

 pair) would render them as completely separate from the rest of 

 their species as they would be on an island, and divergence (as 

 Wallace lias sufficiently shown) would begin. This divergence 

 might progress to a great or a small extent, or even be imper- 

 ceptible, but in any case the new species would be infertile with 

 the species it sprang from. 



If this theory be admitted, we must distinguish between 

 varieties and species by saying that the former arise by spon- 

 taneous variations in various parts of the body, and only gra- 

 dually become mutually infertilBfchus becoming species), while 

 the latter arise sometimes in t^^Bfry, but sometimes by spon- 



taneous variations in the generative elements, and are in this 

 case originally mutually infertile, but only gradually become 

 otherwise divergent. 



I would suggest the following tests, and should be glad of any 

 facts, from experience or from books, which can help in applying 

 them : — 



(1) If this theory is true we ought to find species (incipient) 

 mutually infertile, but not otherwise distinguishable ; and 



(2) We ought to find that island and other isolated species 

 which have arisen not by limited fertility but by geographical 

 instead of physiological separation are often mutually fertile 

 even when as widely divergent as the artificial varieties of dogs 

 or pigeons. Edmund Catchpool 



The Grove, Totley, Sheffield, October 23 



Earthquake Measurement 



In an article on "Earthquakes" in last week's Nature (p. 

 608), Dr. H. J. Johnston-Lavis takes exception to the records of 

 earthquake motion which I have published, on the ground of 

 their complexity, and pronounces the Plain of Vedo unsuitable 

 for earthquake observations. 



Now this seems to me to be a very eclectic way of treating 

 earthquakes. We can measure earthquakes only where we find 

 them, and I suppose the first qualification in a site for an earth- 

 quake observatory is that there should be plenty of earthquakes. 

 The Plain of Yedo possesses this qualification in a very high 

 degree ; and if the disturbances which occur in it are of a very 

 much more complex character than our a priori notions about 

 earthquakes may have led us to expect, it is not the Plain of 

 Yedo that is to blame. 



I fully agree that on a rocky formation the results will be dif- 

 ferent from those I found on an alluvial plain, but the instru- 

 ments and methods which have been successful on the one are 

 just as applicable to the other. The seismometers which have 

 been used in Japan will serve to measure, with equal accuracy, 

 earthquakes of a similar degree of destructiveness in other 

 places, whatever be the nature of the ground. And several of 

 the types already employed need little more than a change of 

 scale in their construction to suit them for such formidable con- 

 vulsions as the Ischian earthquake, to which your correspondent 

 refers. 



In describing and figuring a number of proposed seismographs, 

 Dr. Johnston-Lavis has very frankly disclaimed a technical know- 

 ledge of mechanical construction, and for that reason all minute 

 criticism of his suggestions may be withheld. If however he 

 will refer to the Transactions of the Seismological Society of 

 Japan, or to my "Memoir on Earthquake Measurement," he 

 will see that some of the devices he suggests are not new. The 

 plan of registering the amplitude of a pendulum's motion rela- 

 tively to the earth by making the bob draw up a thread through 

 a hole in a plate fixed below it was used some years ago by Dr. G. 

 Wagener ; and a massive slab free to roll on spherical balls formed 

 in 1876 the seismometer of Dr. G. F. Verbeck. It was re- 

 invented a year or two ago by Mr. C. A. Stevenson, and de- 

 scribed by him before the Royal Scottish Society of Arts. The 

 theory of the apparatus is discussed in §§ 31-32 of my memoir. 

 Dr. Johnston-Lavis's plan of recording the azimuth of a movement 

 by means of numerous electric contacts and "a pile of electro- 

 magnets " is a very retrograde step from the perfectly successful 

 method, used in Japan, of resolving all horizontal movements 

 into components along two fixed directions, these components 

 being independently recorded in conjunction with the time. 



Speaking of the use of the common pendulum as a seismo- 

 meter, the author says that by using a short pendulum we may 

 measure oscillations of short period, and by using a long 

 pendulum we may measure slow earth-tiltings. Almost the 

 reverse of this is the case. A short pendulum acquires, by earth 

 movements of short period, a swing which cannot be distin- 

 guished from the movements we wish to measure, and whose 

 extent depends on the accidental agreement of its period with 

 theirs ; but a short pendulum can be properly used to record 

 slow earth-tiltings, with respect to which it is sensibly dead- 

 beat. A long pendulum can be used to measure short-period 

 movements ; it can also be used (and its only advantage 

 over a short pendulum is greater sensitiveness) to measure slow 

 tiltings. 



For vertical motion Dr. Johnston-Lavis condemns (but with- 

 out giving any reason) my own and another vertical-motion 

 seismograph — which theory and experience agree in proving 



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