October 31, 1895J 



NA TURE 



64^ 



LETTERS TO THE EDITOR. 



f The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 '[ manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications.'\ 



Introduction of a West Indian Frog into the Royal 

 Gardens, Kew. 



A SHOB r lime ago Mr. VV. Watson, the Assistant-Curator of 

 Kew Ciardens, informed me that he had noticed for several years, 

 in some of the hot-houses, specimens of a small frog, which, 

 hiding away during the day among the pots and orchid-baskets, 

 enlivened the quiet evenings with their shrill, whistling notes. 

 Suspecting that this frog must be a foreign importation, I asked 

 the Director to allow some of the specimens to be caught, and 

 some days ago I had the pleasure of receiving three specimens 

 in excellent condition. 



The frog is Hylodcs martinicensis, a small arboreal species, 

 distributed over, and common in, many West Indian Islands 

 (Martinique, Porto Rico, St. Vincent, Dominica, Barbadoes, &c., 

 and possibly in Trinitlad). Mr. Watson recollects that he 

 observed it first some ten years ago, that he lost sight of it for 

 some time, but that it reappeared about four or five years ago. 

 Taking into consideration the few facts with which we are 

 acquainted as to the reproduction of this frog, it "eems most 

 probable that several specimens of both sexes were, on more 

 than one occasion, accidentally introduced in Wardean cases. 



However that may be, it is evident that the frogs ha\'e freely 

 propagated since their introduction. .\t present they are most 

 numerous in the propagating houses, in which the temperature 

 ranges between 80° and 100°, sinking in winter at times to nearly 

 60^. Accompanying Mr. \Vatson one evening, I heard from 

 several points the call of the frogs, which somewhat resembled 

 the jiiping of a nestling bird : and guided by the sound, I had 

 soon the pleasure of seeing one of them clinging to the side of a 

 glass-case. 



There is nothing extraordinary in the accidental importation 

 of individuals of a tropical species of frog into Europe ; but it 

 is an interesting experience, that the species should have 

 (Permanently established itself. This is owing, in the first place, 

 to the favourable conditions under which it found itself placed, 

 and, secondly, to the peculiar mode of its propagation. 



hylodcs martinicensis, and probably the majority of its 

 congeners, does not spawn in water, but deposits from fifteen 

 to thirty ova on leaves in damp places. After a fortnight the 

 young frogs are hatched in a perfect form, having passed through 

 the metamorphosis within the egg, thus escaping the vicissitudes 

 and dangers to which they would have been exposed during the 

 progress of the usual Batrachian metamorphosis. 



This instance of the accliiDatisation in Kew Gardens of 

 the " Coqui "' (as the frog is called in Porto Rico) is unique 

 in Batrachian life at present. I trust that the little guest may 

 long flourish where it has found such a congenial home, and 

 where it usefully aids in the destruction of plant-eating insects 

 and wood-lice, of which I found great numl)ers in the stomach 

 of a specimen. If at a later period a nest with ova were 

 discovered, Mr. Dyer wnuld delight the heart of embrj'ologists, 

 to whom the opportimily of examining fresh ova of this frog 

 would be most welcome. .Vlbert Gi nther. 



Kew, October 20. 



The Cause of an Ice Age. 



1 1 appears to me that the jjosition taken up by Sir Robert 

 Ball in his book, " The Cause of an Ice Age," is seriously mis- 

 represented by Sir II. Itoworth in one paragraph of the criticism 

 wbichappearsin Natiri-; of CJctober 17. .Sirll. Howorth .s.ays, 

 that the fact of the invariability of the ratio of the heal received 

 by our hemisphere in simimer to that received in winter cannot 

 be the cause of variability in climate ; "if, .as we are told in the 

 book over and over again, this particular proportion (63 : 37) is 

 the cause of the Ice age, we must be living in an Ice age now, 

 and we must always have been living in an Ice age." Now it 

 is nowhere asserted by .Sir Robert Ball that the invariability or 

 the magnitude of this ratio is the cause of an Ice age, but it is 

 very clearly explained that he assumes the cause of an Ice age 

 to be a. particular range of positions of the line of equinoxes com- 

 bined with a high value of the eccentricity of the earth's orbit, 



NO. 135;, VOL. 52] 



and that the fact that the above ratio is 63 : 37, and not unity, as 

 appears to have been supposed to Ije the case, is relevant only 

 so far as it inclines us to regard the changes of climate due to 

 the causes just mentioned as much greater than we might other- 

 wise have regarded them. 



It seems obvious that a large value of the eccentricity con- 

 temporaneous with a favourable position of the line of equinoxes 

 will correspond to some change in climate. Whether this cause 

 is a dominant one, or even an important one, in its effect on 

 climate, is of course an open question, and one upon which I 

 express no opinion. Sir H. Moworth thinks that Sir Robert 

 Ball has inadequately recognised the fact that the ratio of heat 

 received in summer to that received in winter by one hemisphere 

 has been calculated by Wiener. I find, however, on page 90 

 (second edition), the following reference to Wieners work. 

 " They depend on the mathematical calculation given for the 

 first time, I believe, by Wiener in his work, " Zcitschrift der 

 Oe-sterreichischen (Jesellschaft fiir Meteorologie,'' vol. xiv., 

 1879, p. 129. . . . .My chief object is to emphasise the relation 

 of these calculations made l)y Wiener to the astronomical 

 theory." Wiener's work is also ntentioned in the preface. 



On the general question as to the adequacy of Croll's theory, 

 with or without the fact which Sir Robert Ball adduces solely 

 with a view of strengthening that theory, I express no opinion ; 

 it seemed to me, however, that in fairness, .some of the remarks 

 made by Sir II. Iloworth required refutation. 



Christ's College, Cambridge. i;. W. HoBSON. 



Green Oysters. 



Only today I was able to read Prof. Lankester's letter 

 (Nature, May 9, 1895), and wish to reply briefly. My note in 

 Monitorc Zoologico was simply a preliminary communication ; the 

 proofs of my asserti^)ns will be given /';/ extenso in a pa]>er which 

 will soon he published. My conclusions in that part which may 

 interest the previous labours of Prof. Lankesler may be briefly 

 expressed as follows : — 



(i) My observations have always been made on true huitres 

 de Marennes. 



(2) I believe that Prof. Lankester must have overlooked the 

 recent works on the histology of Molluscs by Janssen, Rawitz 

 and others, or he would have seen that his "gland cells" are 

 the becherzellcn, cellules caliciformes of the authors quoted ; 

 which are inside the branchial ejiithelium, and not on its surface, 

 and never can be considered wandering, nor can they have 

 amoeboid movements. It would be strange, therefore, to con- 

 sider such " gland cells " .as similar to the amcebocytes of the 

 blood ! 



(3) Prof. Lankester says that the "gland cells " contain green 

 granules in the Marennes oysters, but this is entirely due to an 

 opt'cal illusion ; if one examines a fresh piece of branchial 

 lamella of tlie gr^en huitre de Marennes, the "gland cells " 

 appear green, but if these cells be separated from the epithelium, 

 one finds that they are always colourless, and that they appeared 

 green because they are surrounded with green matter. Making 

 careful sections of the brancliial lamella; or the labial |xilps, one 

 finds clearly : {a) that the gland cells are never green ; (b) that 

 the superficial epithelimn is green : (c) that some amcebocytes 

 and large masses included in the epithelium are also green. I 

 am ready to furnish Prof. I.;inkester with microscopical pre- 

 parations showing what I assert. 



(4) The green of the Marennes oysters is not a hurtful sub- 

 stance which must be got rid of, and it is incorrect to imagine 

 a defensive phagocytosis performed by amiebocytes. To me it 

 is quite obvious that the green colouration is merely due to a 

 true assimilation of nutritive substance which takes place through 

 the agency of the epithelium in some jwrtions of the intestine 

 and in the branchial lamella;-. .\nd no doubt it is the amrebocytes 

 who carry the green substance, assimilated from the epithelium, 

 to the liver. I am quite aware that these results of my researches 

 are new, and it is for this reason that in communicating them to 

 the Monitorc Zoologico I noted that they are of some importance 

 to our further knowledge of the iihysiology of mollusca. 



(5) It is a mistake to believe that the oysters .are green because 

 they feed on Navicula ostrearia : the truth is that the alga is 

 green for the same rea-son that the Marennes oysters are so, 

 which is from the nature of the pares and claires bottom. It is 

 therefore the ,s.ame substance, viz. the blue pigment " Marcnnin," 

 which is found in both. 



(6) The chemical |>art of my work is not concluded, and I 



