7 6 



NA TURE 



[July 15, 1922 



Letters to the Editor. 



\Tlie Editor docs not hold Siimsclf responsible for 

 opinions expressed by Ins 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.] 



Interspecific Sterility. 



The implications of modern genetics have been so 

 little considered by biologists in this country that the 

 criticism of my address by Dr. Cunningham (Nature, 

 June 17), though in purpose destructive, is not 

 unwelcome. Of the points he raises one chiefly calls 

 for reply. I directed once more the attention 

 of naturalists to the fact that we still await the 

 production of an indubitably sterile hybrid from 

 completely fertile parents which have arisen under 

 critical observation from a single common origin. 

 So far as our knowledge goes, all the domesticated 

 races — for example, of dogs, of pigeons, of fowls 

 among animals; and of cabbages, of peas, of Primula 

 sinensis, and many more among plants — when inter- 

 crossed among themselves never produce this sterility 

 in their mongrels, though the races are often distinct 

 enough to pass for species. But if we begin crossing 

 natural species, even those which on our reckoning 

 must be very closely allied, we constantly find either 

 that they will not interbreed, or that, if they can be 

 crossed, the result is more or less sterile. Dr. Cunning- 

 ham takes exception to my speaking of this inter- 

 specific sterility as the chief attribute of species, 

 but he will not dispute that it is a chief attribute of 

 species. 



The races of fowls might, as he holds, on account 

 of their enormous divergences, be without impro- 

 priety compared to natural species. They may 

 also, as he thinks, all descend from Gallus bankiva 

 (though I find that difficult to believe) ; but inasmuch 

 as they do not show interspecific sterility they do not 

 help us to understand how that peculiar property 

 of species arose in evolution. In contemporary varia- 

 tion we witness the origin of many classes of differ- 

 ences, but not this ; yet by hypothesis it must again 

 and again have arisen in the course of evolution of 

 species from a common ancestry. The difncultv is 

 no new one ; but I emphasised it because naturalists 

 should take it more seriously than they have done 

 hitherto. Especially now that a great deal of 

 experimental breeding is in progress, watch should 

 be kept for such an occurrence. I by no means 

 declare that the event cannot happen, but, so far as 

 I know, it has not been witnessed yet. 



Dr. Cunningham tries to fill the gap by adducing 

 two instances. The first is that of Oenothera gigas. 

 Now I had not forgotten the tetraploids, which so 

 often do not breed freely with diploids, but the 

 applicability of that example is exceedingly doubtful. 

 Interspecific sterility or incompatibility may well be 

 a consequence of nuclear diversity, though we can 

 scarcely regard an unresolved pair of twins, such as 

 the tetraploid must be, as a specifically distinct 

 organism. 



His second illustration, if authentic, would be 

 more nearly what is wanted. He says that " two 

 mutants of Drosophila in Morgan's experiments are 

 almost completely sterile with one another." The 

 allusion is probably to a paper of Metz and Bridges 

 (Proc. Nat. Acad. Sci., 1917, iii. p. 673), in which they 

 claimed to have found two mutants of D. virilis 

 which gave partially sterile hybrids when intercrossed. 

 Dr. Cunningham is not perhaps aware that this claim 



NO. 2750, VOL. I io] 



was afterwards withdrawn (Metz, ibid., 1920, vi. 

 p. 421), inasmuch as one of the mutants was found to 

 have been partially sterile. Metz and Bridges did 

 adduce another example in D. ampelophila, but for 

 a variety of reasons that, even if substantiated, would 

 scarcely be to the point. As a matter of fact, how- 

 ever, in so far as opportunity of repeating the 

 cross has occurred, complete fertility resulted. I 

 know of no other example to which Dr. Cunningham 

 can be referring. 



Mr. Crowther (Nature, June 17, p. 777) mistakes 

 my meaning. It is, as he says, not difficult to 

 " imagine " interspecific sterility produced by a 

 gradual (or sudden) modification. That sterility may 

 quite reasonably be supposed to be due to the in- 

 ability of certain chromosomes to conjugate, and Mr. 

 Crowther's simile of the sword and the scabbard may 

 serve to depict the sort of thing we might expect to 

 happen. But the difficulty is that we have never 

 seen it happen to swords and scabbards which we 

 know to have belonged originally to each other. On 

 the contrary, they seem always to fit each other, 

 whatever diversities they may have acquired. 



W. Bateson. 



July 2, 1922. 



Geology and the Nebular Theory. 



I do not deserve the reproaches of Prof. Coleman 

 (Nature, June 17, p. 775). My molten earth did 

 not, in point of fact, owe its thermal energy to 

 primitive condensation, but to accumulated radio- 

 active heat, as the concluding part of my lecture 

 might have shown. I am committed neither to the 

 nebular theory nor to the planetesimal theory. 



Had I adopted the wider definition of the Archaean 

 favoured by Prof. Coleman, I could not, of course, 

 have described the Archaean sediments as scanty. I 

 referred to an Archaean limited to the Keewatin, 

 and to the Laurentian outpourings of granitic 

 materials. The Keewatin is generally described as 

 mainly volcanic in origin. The definition of Archaean 

 and Algonkian favoured by Van Hise and Leith 

 (Bulletin 360 of the United States Geol. Survey) would 

 bear me out. 



While many geologists would agree with Prof. 

 Coleman as to his estimates of Archaean sediments, 

 many, I think, will disagree with him in his contention 

 that there was nothing exceptional in the thermal 

 conditions attending the Laurentian revolution. 

 Prof. Coleman's most interesting discovery of an 

 ice age in Huronian times has, I submit, nothing 

 to do with the matter. On the other hand, I think 

 geologists in their interpretation of the Archaean 

 should keep in mind the possibility (or probability) 

 that the phenomena observed are due to paroxysmal 

 thermal developments traceable to deep-seated radio- 

 active substances : and that these developments, 

 which appear to have been world-wide in extension, 

 may have been sufficiently intense to have closed a 

 biological era. So that, in fact, we have in the 

 Archaean the almost obliterated record of a prior 

 geological age. J. Joly. 



Trinity College, Dublin. 



I have read with interest Prof. Coleman's timely 

 reminder, in Nature of June 17, p. 775, of the 

 essentially intrusive relations of the Archaean and of 

 the frequently made deduction that the oldest visible 

 rocks of the earth's surface are sedimentary. Of 

 course this deduction is perfectly sound, provided the 

 age of an intrusive rock is taken, as has been the 



