i02 



NATURE 



[December 3, 1891 



them out because we know what a stumbling-block even 

 slight mistakes are to such students. Their reverence for 

 printed results is often wonderful. The utility of Mr. 

 Lock's " Higher Trigonometry " is greatly hindered by the j 

 number of typographical blunders. i 



The Physical Geology and Geography of Irelaftd. By ' 

 Edward Hull, F.R.S. Second Edition. (London: 

 Edward Stanford, 1891.) 

 The first edition of this book was reviewed in Nature 

 rather more than thirteen years ago (vol. xviii. p. 354). 

 Of the second edition, which will be welcomed by all 

 students of the subjects it deals with, we need only say 

 that Prof. Hull has embodied in it the additions which 

 have lately been made to our knowledge of the geo- 

 logical structure of Ireland. The more important of 

 these additions he sums up under the following heads :— 

 (i) The determination of the occurrence of Archeean 

 rocks in certain districts of the west and north of Ireland. 

 (2) The determination of the peculiar relations subsisting ^ 

 between the Lower Devonian (or Devono-Silurian) strata | 

 and the Upper Old Red Sandstone and Carboniferous 

 series of the southern districts. (3) Additional evidence 

 regarding the relative ages of the trachytic and basaltic I 

 lavas of Antrim. (4) Evidence of the invasion of Ulster 

 by a great ice-sheet from the Grampian Mountains 

 of Scotland during the earliest stage of the Glacial 

 period. I 



The Ouse. By A. J. Foster, M.A. (London : Society ^ 



for Promoting Christian Knowledge, 1891.) 

 In this little book the course of the Ouse is traced from 

 its source to the point where it enters the sea, and some 

 account is given of the various elements of interest that 

 are to be met with on the way. The idea is good, and 

 the author has worked it out skilfully. Any boys or girls 

 who may read the volume will find at the close that 

 they have obtained from it much sound geographical 

 knowledge. i 



LETTERS TO THE EDITOR. 



[ 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 ^ArtVK^. 

 No notice is taken of anonymous communications. ] 



A Difficulty in Weismannism. 



In your number of October 29 (vol, xliv. p. 613), Prof. 

 Hartog depicts a dilemma in which a study of Weismann's 

 theories has placed him. 



Prof. Hartog sums up the main points of Weismann's theories 

 in five theses, but, considering the great importance which the 

 latter attaches to the operation of natural selection, he might 

 well have added a sixth to the list. 



There can be no doubt that, of the two hypotheses brought 

 forward in the letter, hypothesis B is the one adopted by 

 Weismann for the explanation- of the problems of heredity. 

 We are therefore not concerned with hypothesis A. 



"According to hypothesis B," Prof. Hartog states, "the 

 Ahnenplasmas of all Metazoa being similar and Protozoan, if 

 the numbers are eqtial and the shuffling fair, any two parents 

 may beget any offspring whatever ; . . . a lioness might be 

 expected to bring forth a lobster or a starfish, &c." 



What does Prof Hartog mean h-^ fair shuffling ? Surely not 

 such shuffling as is resorted to in the game of whist, but such 

 shuffling as he himself describes in thesis 4. He states here 

 that the "process is comparable to the shuffling of two packs of 

 cards by taking half from each and. joining the talons or 

 remainders to form a new pack." 



It surely cannot be imagined that Weismann ever intended to' 

 assert that with each sexual act there was a rearrangement of 



NO. II 53, VOL. 45] 



the Ahnenplasmas comparable to the shuffling of a pack of card& 

 during the game of whist. 



Did he anywhere asert this, we should naturally expect him 

 to believe that a lioness might as well bring forth human beings 

 or lobsters as noimal cubs. 



With the evolution of sexuality, the excessively numerous 

 Ahnenplasmas of our variable Protozoan ancestors became 

 arranged in more and more complex, ever-varying cojiibinations. 

 At the very outset, natural selection operated. The variations 

 (due to the combinations) most advantageous to the species were 

 perpetuated. Unfavourable variations mvolved extinction. It 

 is the special combination of the units of ancestral germ-plasm 

 which predetermines the structure of the mature individual. 

 This combination, of course, is very closely related to the two 

 combinations from which it arose, and it is just this closeness of 

 relationship which prevents us from supposing that a lioness 

 can ever produce anything but cubs. Changes in the com- 

 binations are only slowly ctt'ected. The influence of the mother 

 is due to the fact that one-half of the maternal combination is 

 present in the offspring, aud similar .^-tatements can of course be 

 made concerning the influences of father, grandfather, great- 

 grandfather, &c. 



Do, not these two considerations — (i) that the nature of the 

 individual depends upon the peculiar combination of units of 

 ancestral Protozoan germ-plasm, a combination very closely 

 related to two previous ones (ow ing to the fact that, in sexual 

 union, two halves of immediately preceding combinations are 

 united to make one whole) ; (2) itiat the operation of natural 

 selection provides for the extinction of useless, and \.\\t preserva- 

 tion of useful variations — afford to Prof. Hartog the means of 

 escaping from his dilemma? A. H. Trow. 



Penarth, Cardiff, November 14. 



The contributions of Mr. Trow and Dr. Poulton to this 

 discussion render necessary an explanation that should, perhaps, 

 have accompanied my first letter. After rough-drafiing this, I 

 felt misgivings lest I might have misconceived Weismana's 

 meaning, and set up a man of straw to knock down. Accordingly, 

 I wrote to Prof. Weismann to ask if 1 riyhtly understood his 

 meaning, explaining uiy object in doing so ; and he answered 

 my queries with great kindness, courtesy, and fulness. As I 

 wrote back to him, I then thought it better, relieved from my mis- 

 givings, to state the point without reference to his letter. But 

 Mr. Trow and Dr. Poulton have both blamed my use of the 

 word shuffling, and appear to think that my hypothesis A is a 

 purely imaginary conception of the straw man order. I hope, 

 therefore, 1 shall not be accused of having wilfully kept a trump 

 card up my sleeve if 1 now quote the two esential passages of 

 Prof Weismann's letter, which were written in definition of the 

 points at issue. 



" Ich denke mir dass das Keimplasma eines Individuum's aus 

 einer gewissen Zahl von Einheiten besteht, welche untereinander 

 sehr ahnlich, aber nicht gleich sind. Die Unterschiede zwischen 

 ihnen entsprechen meistden Unterschieden zwischen je zwei Indi- 

 viduen derselben Species. Jedes derselben wiirdeim Stande sein 

 ein Individuum der Art hervorztibringen falls es sich zu der dazu 

 nbthigen Masse vervidfdltigen kdnnte oder wiirde." The 

 sentence I have italicized corresponds, I think, very fairly to my 

 hypothesis A: "Each Ahnenplasma unit corresponds to an 

 individual of the species itself; and if put under suitable trophic 

 conditions would, singly, reproduce such an individual." Dr. 

 Poulton writes : " I agree with Prof Hartog in considering it 

 [Hypothesis A] as valueless." I am far from considering any 

 hypothesis as valueless which upsets a wrong theory of which it 

 should be the mainstay. 



Prof. Weismann goes on: "Sie konnen ganz wohl die 

 geschlechtliche Fortpflanzung mit dem Mischen eines Kartenspiels 

 vergleichen, aus dem immer dieHiilfte der Karten entfernt wird, 

 Nur ist nicht zu vergessen dass die Karten selbst nicht vollig 

 unveranderlich sind." It is obvious that Prjf Weismann 

 accepts the peculiar mode of shuffling I have described (not the 

 ordinary mode at whist), as a fair illustration of his conception 

 of fertilization and its antecedents. He always speaks of com- 

 binations \x\ his " Essays," and ■ro\. permutations. The reason is 

 obvious : the figured elements of the living nucleus are constantly 

 changing their relative position ; and it is these that are the 

 outward and visible sign of the mysterious ancestral units. 



