Apru 1 8, 1878] 



NATURE 



483 



should be able to take a photograph of the eclipse, in the 

 third order, in two minutes. Similarly, we may hope for 

 a photograph of the second order in two minutes, and it 

 is, I think, highly probable also that a photograph of the 

 first order may be obtained in one minute. To make 

 assurance doubly sure, the whole of the totality may be 

 used during the coming eclipse, but if there be several 

 such attempts made it will certainly be worth while to try 

 what a shorter exposure will do. 



Now, by mounting photographic plates on both sides 

 of the axis, one solidly mounted equatorial of short focal 

 length may enable us to obtain several such photographs, 

 with varying lengths of exposure. I insist upon the 

 solidity of the mounting because, if any one plate is to 

 be exposed during the whole of totality, the instrument 

 must not be violently disturbed or shaken while the 

 eclipse is going on. I think, however, it is quite possible 

 to obtain more than one photograph of the lower order 

 spectra without any such disturbance in this way. The 

 same plate may be made to record three, or even four, 

 exposures in the case of the first order in an eclipse of 

 four minutes' duration, by merely raising or lowering it 

 after a given time, by means of a rapid screw or other 

 equivalent contrivance, so that a fresh portion of the 

 same plate may be exposed. Similarly, the plates on 

 which the spectra of the second order are to be recorded 

 may be made to perform double duty. 



If one equatorial thus mounted were to be devoted to 

 each quadrant of the coronal atmosphere, it is certain, I 

 think, that most important results would be obtained. 



J. Norman Lockyer 

 (Tk? be continued?) 



GIGANTIC LAND-TORTOISES 

 Gigantic Land-Tortoises, Living and Extinct, in the 

 Collection of the British Museum. By A. C. L. G. 

 Gunther, M.A., M.D., F.R.S. Keeper of the Depart- 

 ment of Zoology. (London : Printed by Order of the 

 Trustees, 1877.) 



THE recent and extinct gigantic land-tortoises in the 

 collection of the British Museum has just received 

 at the hands of Mr. A. C. L. G. Gunther, Keeper of the 

 Department of Zoology, an elaborate and exhaustive 

 memoir and history. As early as 1872 Dr. Giinther had 

 made much progress in the elucidation of their structure, 

 but in 1874 the osteology of the Mascarene tortoises 

 had still more engaged his attention. Again in 1877 new 

 matter arising from fresh materials imported into Eng- 

 land from the Aldabra group of islands, Mauritius, 

 Rodriguez, and the Albemarle and Abingdon Islands, 

 enabled Gunther to complete his memoir upon these 

 gigantic land-tortoises, recent and fossil. 



This important volume contains a description of the 

 raqes of the Aldabra group, the extinct races of the Mas- 

 carene group (Mauritius and Rodriguez), and lastly, the 

 Galapagos Islands races. Dr. Gunther, at p. 10, gives a 

 synopsis of the fossil and living gigantic land-tortoises. 

 He bases his classification upon the presence or absence 

 of the nuchal plate— frontal portion of the skull — 

 condition of the pelvis as to nature of the symphysial 

 bridge, and whether the gular plate is single or double. 

 The Aldabra tortoises, or those of the Aldabra Islands, 



fall under the fii'st gf cup, or those with the nuchal plate 

 present, gular plate double, and frontal portion of skull 

 convex and with the pelvis having a narrow symphysial 

 bridge. Four species of Testudo, all living, occur in the 

 Aldabra group. 



The second group, embracing the Mascarene and 

 Galapagos tortoises, possess no nuchal plate ; the sym- 

 physial bridge is broad, and the frontal portion of the 

 skull is flat. The Mascarene species, four in number, 

 are all extinct, and are found by Giinther to have a single 

 gular plate and short sternum, whereas the Galapagos 

 tortoises have a double gular plate and rather large 

 sternum, and all but one species {Testudo ephippiutn), 

 from Indefatigable Island, are living. 



These deductions arrived at by Dr. Giinther after 

 years of long and patient labour, greatly add to our 

 knowledge of the structure of the Testudinas greatly 

 removed in space ; he not only shows that the Aldabra 

 species have definite and almost individualised structure, 

 but that they are entirely different species from their 

 nearest or Mascarene neighbours, a great fact in the 

 distribution of life, over an area once continuous land, 

 but now known to be one of depression, and yet geo- 

 graphically contiguous, the Island of Madagascar only 

 separating them. Here, however, we have not a 

 wide distribution in space, and yet no species seems 

 common to the Mascarene and Aldabra Testudinie — 

 the living races of the Aldabra group being entirely 

 different from the extinct races of the Mascarenes. Dr. 

 Giinther endeavours to show that in the absence of 

 direct genetic relationship between the tortoises of the 

 Galapagos Islands and the Mascarenes, that some " terres- 

 trial tortoises" were transported through some agency 

 (" stream or current ") from the American continent to 

 the Galapagos — and similarly that those of Madagascar 

 or Africa migrated in a similar manner to the Mascarenes. 

 The origin and geographical distribution of species espe- 

 cially terrestrial is always of the highest interest to earnest 

 students of life in its various phases. The history and 

 origin of species, and their distribution, is perhaps one of 

 the most difficult problems now engaging the minds of 

 naturalists, and Gunther refers to the reappearance of 

 the "Indian, Mascarene, and Aldabra gigantic land- 

 tortoises in the Galapagos," as one of these — not, he says, 

 in " typical singularity, but with all the principal secon- 

 dary modifications reproduced." The greater extension 

 of this large Chelonian type at a former geological epoch 

 seems manifest, when we find remains at Malta 

 corresponding with those of the Galapagos tortoises, 

 and the close affinity between the Galapagos and 

 the Aldabra and Mascarene species, although separated 

 by so vast a distance; we must grant a continuity 

 of land over the region now covered by the Pacific, 

 and which for ages has undergone, and is still 

 undergoing depression. No one can doubt or fail 

 to see the great changes that have taken place 

 in the physical geography of South Africa, whose 

 attenuation towards the south and eastern coasts is due 

 to depression, thus causing the isolation of Madagascar, 

 the Mascarene Islands, and the Seychelles, such severance 

 and island making, through causes long-continued and 

 not equally the same areally in equal times, has produced 

 that specialised or peculiar fauna for which many of 



