December 23, 1922] 



NA TURE 



843 



Interspecific Sterility. 



Dr. Bateson's letter on interspecific sterility in 

 Nature of July 15, p. 76, has given rise to an interest- 

 ing discussion in later issues, which may be sum- 

 marised thus : — 



Sterility between wild species is not nearly so 

 common as was formerly supposed, yet it undoubtedly 

 occurs frequently, both between species with the same 

 number and with different numbers of chromosomes. 

 The cause of this sterility has not yet been made out 

 with any degree of certainty. On the other hand, 

 crosses between domestic races are, almost exclusively 

 at least, perfectly fertile, although Dr. Gates rightly 

 points out that sterility may often be expressed in 

 lethal factors and that lethai factors are of common 

 occurrence in Morgan's " domestic " races of Droso- 

 phila for instance. 



Dr. Bateson's starting - point is his belief, that 

 domestic races as well as species in Nature have 

 arisen by some process of transmittable variability, 

 let us say by mutation. At least, on no other 

 assumption can I explain his sentence (I.e. p. 76) : 



In contemporary variation we witness the origin 

 of many classes of differences, but not this (e.g. inter- 

 specific sterility) ; yet by hypothesis it must again 

 and again have arisen in the course of evolution from 

 species of a common origin." 



Geneticists are aware that this view is not mine. 

 According to my view two genotypically different 

 gametes are required to give rise to new forms : 

 domestic races as well as natural species arise by 

 crossing. If this is the case — and nobody will deny 

 that, at least in the production of " races," crossing 

 plays a most important role — there is no cause to 

 assume that sterility has ever " arisen " from fertility 

 in the course of evolution. We have, for the present, 

 to be satisfied with the establishment of the fact that 

 some gametes, differing in constitution, after crossing 

 give rise to wholly or partly sterile progeny, while 

 others give fertile progeny only. 



As there is no reason to assume that our domestic 

 products are the result of crosses only of such wild 

 species as from the start gave exclusively fertile 

 progeny — although, as we shall see, such crosses may 

 indeed have been favoured — it follows that the general 

 inter-racial fertility of domestic products must have 

 been " acquired." Consequently the problem under 

 discussion is not how sterility arose from fertility, but 

 how a form-group 111 which both inter-racial fertility 

 and sterility occurred, became changed into one, the 

 members of which were all interfertile. 



It seems to me that the most simple explanation is 

 offered by the assumption that man from the begin- 

 ning, for example, from the initial cross or crosses 

 among his animals or plants taken from Nature, in 

 an attempt to domesticate them, has selected the most 

 fertile forms and has continued to do s6; in other 

 words, that he has persistently exterminated those 

 forms which were intersterile and kept only those 

 which were interfertile. 



While at the present moment intersterility of 

 domestic races might offer considerable advantages, 

 allowing, for example, the cultivation side by side of 

 different varieties of flowers without fear of crossing, 

 there was no such advantage at the very beginning 

 of domestication, when the only object was not to 

 obtain a particular kind but any kind of domestic 

 animal or plant. By this continued selection of 

 interfertile forms, man himself has by now cut off 

 the possibility of obtaining intersterile races. 



The following case may illustrate my meaning : 



According to my view, our domestic races of fowl, 

 which " without impropriety may, on account of 



their enormous dinerences, be compared to natural 

 species," have arisen from a cross in which more than 

 one wild species has taken part. Prof. Ghigi, the 

 well-known ornithologist of Bologna, is of the same 

 opinion, and Dr. Bateson also evidently looks favour- 

 ably on this view, as he states that he finds it difficult 

 to believe that all races of poultry should have 

 descended from Callus bankiva only. While all races 

 of domestic poultry are, so far as is known, fertile 

 inter se, crosses of Gallus bankiva and G. Sonnerati, or 

 of the former and G. varians, give rise, as is well known, 

 to a partly fertile and partly sterile progeny, so that, 

 if our domestic fowl have really arisen from crosses of 

 these wild species, their inter-racial fertility was not 

 primitive, but " acquired " by elimination of the 

 sterile stock. 



Thus, according to the views here stated, the 

 starting-point in the formation of domestic races as 

 well as of natural species was the same, to wit, a cross. 

 In those cases in which the product of such a cross 

 was a sterile hybrid, the attempt to originate new 

 races or species was smothered in its birth. Such 

 crosses as gave perfectly interfertile progeny were 

 most acceptable to man, and the cause of the fact 

 that only a very small percentage of the wild species 

 in existence has taken part in the formation of our 

 domestic products may very well be man's partiality 

 for such ab initio fertile crosses. 



In those cases in which intersterile and inter- 

 fertile forms arose from a first cross, man selected the 

 interfertile forms, and so obtained the same kind of 

 starting-point for his further efforts as when the first 

 cross had been perfectly fertile from the beginning. 



The obtaining of well-defined races from such an 

 interfertile crowd could be attained in one way only, 

 namely, by isolation, and we know that isolation is 

 the alpha and omega of successful breeding. 



" Species "-formation in Nature also started from a 

 cross, and Nature's only means of obtaining well- 

 defined form-groups, for example species, also con- 

 sisted in isolation. Ready-made isolation was pre- 

 sented to Nature bv the intersterile forms arisen front 

 a cross ; hence these were favoured, and this accounts 

 for the great percentage of intersterile species in 

 Nature. 



To summarise : The starting-point in the forma- 

 tion of races by man and in the formation of species 

 by Nature is the same, namely, a mixed stock of 

 interfertile and intersterile forms arising from a cross. 



.Man selected the interfertile, Nature the inter- 

 sterile forms, hence the difference in mutual fertility 

 between domestic races and natural species. 



Sterility between species, according to this view, 

 therefore, did not arise from fertility but is the direct 

 result of crossing. J. P. Lotsy. 



Velp. November 28. 



Occult Phenomena and After-images. 



If the hand be held against a dark background in 

 a very subdued light, coming from behind the observer 

 and falling on the hand, a diffuse glow will be observed 

 round thumb and fingers, frequently uniting the 

 finger tips. A little patience and a moderately clean 

 hand are all that is required to observe the pheno- 

 menon! 



Further, however, if a hand be cut out of white 

 cardboard (which is easily done by placing the hand, 

 with thumb and fingers moderately spread, on the 

 cardboard, tracing the outline in pencil, and cutting 

 round with scissors) and feebly illuminated in the way 

 described, a similar but somewhat stronger glow will 

 be observed. In the case of both the flesh and the 

 cardboard the shape of the glow can be modified by 

 slow movement of the hand. 



NO. 2773, VOL. I IO] 



