124 



NATURE 



{June 7, 1883 



secondly, because we almost always close part of the slit of the 

 spectroscope « ith a shutter, so that the image is cut off sharply 

 by the shadow of the shutter. Strong lines extend into the 

 shadow more or less, and if there is a real reversal the extension 

 of the reversed part into the shadow is trumpet shaped, whereas 

 if it is only a pseudo-reversal it is closed." 



1 beg to be allowed to call attention to one or two points in 

 the above quotation which I imagine may lead to a misconcep- 

 tion of the phenomena observed, and of my remarks thereon. 



First, as regards over-exposure, it is assumed that solarisation 

 is an equivalent for this expression. This is the case only when 

 speaking of the cause, but the word has been used by pho- 

 tographers for many years to describe the effect of over-exposure. 



In all collodion processes, wet or dry, this effect is an undue 

 intensity of the high lights and an overpowering of the interme- 

 diate tints and delicate shadows adjoining them. This appears to 

 be due to the fact that from the intensity of the light not only the 

 direct rays, but those reflected from the back of the glass plate, 

 or even tho^e which are scattered, have sufficient power to act 

 upon the sensitive film. In photographs of spectra this is seen 

 in the nimbus or halo surrounding the strongest metallic lines, 

 which disguises their form. It is well illustrated by my photo- 

 graphs of the magnesium, cadmium, and other spectra, published 

 m the Journal of the Chemical Society, vol. xli , Transactions, 

 1882, p. 90. 



Although I have worked with dry platts of almost every 

 description, and with some modifications prepared by myself 

 which have never been described, I do not recollect having 

 observed that over-exposuie causes any other effect than a too 

 dense deposit of silver, excepting when the vehicle for the sensi- 

 tive salt is a film of gelatine. As far as my experience goes, it 

 is a property peculiar to gelatine pktes, that with such extreme 

 facility they are incapable of development after too strong an 

 actii n of light, and I carefully avoided the term solarisation, 

 since it has been used to describe an effect so different from that 

 to which I desired to call attention. 



Secondly, with regard to difficulty in distinguishing reversals, 

 the sentence above does not exactly represent my experience, 

 and I think it may be seen by those who read my communica- 

 tion, that any want of distinction between real and pseudo- 

 reversals had reference only to photographs which had been 

 already taken with a fixed period of exposure, and that I advo- 

 cated a method of comparative exposures as neces-ary in the 

 study of spectra. It appears that this is one of the means w here- 

 by Profes-ors Liveing and Dewar are able to draw distinctions 

 between real and pseudo-reversals. Thesecond method, namely, 

 the use of a shutter, is extremely useful in observations on arc 

 spectra, which have been so completely studied by them. 1 have 

 been studying s| ark-spectra exclusively, and have not been 

 giving special attention to reversals, in fact, endeavouring as far as 

 possible to avoid them. The use of a shutter does not commend 

 itself to me, since it would cut off a highly characteristic feature 

 in spark-spectra which it is desirable to observe, namely, the 

 extension of the lines, but I may here mention that a speck of 

 dust on the slit, or a fine wire stretched across it, will answer 

 the same purpose as a shutter, without obscuring any consider- 

 able portion of the spark, and may be conveniently employed. 

 And now permit ire to add one word : the same alteration in 

 the intensity of the spark which results in real reversals also 

 frequently causes pseudo-reversals. Sometimes simply a turn of 

 the screw attached to the spring of the contact-breaker on the 

 induction-coil is sufficient to effect this change. 



W. N. Hartley 



Koyal College of Science, Dublin, May 18 



The Northern Zoogeographical Regions 



The facts of zoogeography are so involved, and often appar- 

 ently contradictory, that a skilful dialectician with the requisite 

 knowledge can make a plausible argument for antithetical postu- 

 lates. Prof, lleilprin, being a skilful dialectician and well informed, 

 has submitted a pretty argument in favour of the union of the 

 North American or " Nearctic " and Eurasiatic or " Palsearctic " 

 regions (Proc. Acad. Nat. Sc. Phila., 1882, pp. 316-334, and 

 Nature, vol. xxvii. p. 606), but Mr. Wallace has, with perfect 

 justness it seems to me, objected to his proposition (Nature, 

 vol. xxvii. pp. 482, 483). As Prof. Heilprin's arguments have 

 not been entirely met, however, permit me to submit some 

 further objections to his views. 



Prof. Heilprin has contended "(1) that by family, generic, 



and specific characters, as far as the Mammalia are concerned, 

 the Nearctic and Paloearctic faunas taken collectively are more 

 clearly defined from any or all of the other regions than either 

 the Nearctic or Palasarctic taken individually ; and (2) that by 

 the community of family, generic, and specific characters the 

 Nearctic region is indisputably united to the Pala?arctic, of which 

 it forms a lateral extension." 



Prof. Heilprin has formulated these conclusions after a sum- 

 mary of the families and genera common and peculiar to the 

 regions in question. 



As to families, Prof. Heilprin has presented the following 

 figures : — 



All. Peculiar. 



Nearctic 26 ... I 



Palaearctic 36 ... o 



Oriental 36 ... 3 



Australian 22 ... 8 



Ethiopian 44 ... 9 



Neotropical 31 ... 8 



The proportions of peculiar genera to the entire Mammalian 

 faunas of the several regions are stated to be as follows : — 



All. Peculiar. Percentage. 



Nearctic 74 ... 26 ... 35 



Palasarctic ... ico ... 35 ... 35 



Oriental 118 ... 54 ... 46 



Australian ... 70 ... 45 ... 64 



Ethiopian 142 ... 90 ... 63 



Neotropical ... 131 ... 103 ... 78 



The question may naturally recur why the line which separates 

 "regions" from "subregions" should be drawn between 35 

 and 46 per cent, rather than between 46 and 63 or 64 per cent., 

 or even between 64 and 78 per cent. Prof. Heilprin has not 

 told us why, and I am unable to appreciate the reason therefor. 

 Surely it is not sufficient to answer by simply asking the ques- 

 sions put in Nature (p. 606). 



But an analysis of more (but only approximately) correct 

 figures and a more logical classification of mammals than that 

 adopted by Prof. Heilprin reveal facts materially contravening 

 the tabular statements of that gentleman. 



First we must exclude the marine mammals, because their 

 distribution and limitation are determined by other factors than 

 those which regulate the terrestrial ones. A consideration then 

 of the terrestrial forms leads to the following results : — 



The Arctamerican or Nearctic region has 27 families, of which 

 1 1 are not shared with Eurasia and 4 are peculiar ; it has 68 

 genera, of which 45 do not enter into Eurasia. 



The Eurasiatic or Patearctic region has 32 ] families, of which 

 17 are excluded from North America, and it possesses 89 ' 

 genera, of which 60 have failed to become developed in 

 America. 



Such contrasts will more than compare generally with those 

 existing between Eurasia and India, and even between the 

 " Triarctic " or " Holarctic " and Indian "regions," and the same 

 destructive process by which the northern regions are abrogated 

 would entail the absorption of the Indian as well into a hetero- 

 geneous whole. The three can in fact be well united (as Caeno- 

 gasa), and contrasted with a group (Eogaia) consisting of the 

 African, South American, and Australian regions, as I long ago 

 urged (Ann. and Mag. Nat. Hist. (4), xv. 251-255, 1875), 

 but the claims of each to be considered as " regions " or realms 

 is not thereby affected. Theo. Gill 



Smithsonian Institution, Wa-hington, May 12 



Deductive Biology 



Though no writer has yet afforded any remarks in criticism 

 of Prof. Thiselton Dyer's " word of warning " to biologists, 

 given in Nature, vol. xxvii. p. 554, it does not, I think, follow- 

 that the objection raised by him is to be accepted as unanswer- 

 able. As no one of authority in such matters appears to be 

 forthcoming, perhaps one who can lay no claim to being heard 

 may still be permitted to venture to doubt the validity of the 

 objection as given forth in such emphatic terms by Prof. Dyer, 

 and to point out that most, if not all, of the scientific conclu- 

 sions of importance, especially those accomplished during the 

 present generation, have been arrived at mainly by means of the 

 deductive method. 



1 These [are ;the groups admitted by Prof. Heilprin, 'exclusive of the 

 Pinnipeds. 



