April 29, 1922] 



NA TURE 



555 



matter seems to have escaped him.^ He is at great 

 ; ains to prove that inter-specific crosses are not always 



terile, and he shows that crosses between forms which 

 pass for distinct species may produce hybrids which 

 range from complete fertiUty to complete sterility. 

 The fertile hybrids he claims in support of his argument. 

 II species arose from a common origin, clearly they 

 sliould not always give sterile hybrids. So Darwin is 

 oncerned to prove that such hybrids are by no means 

 always sterile, which to us is a commonplace of every- 

 day experience. If species have a common origin, 

 where did they pick up the ingredients which produce 

 this sexual incompatibihty ? Almost certainly it is a 

 variation in which something has been added. We 

 have come to see that variations can very commonly — 

 I do not say always — be distinguished as positive and 

 negative. The validity of this distinction has been 

 doubted, especially by the Drosophila workers. Never- 

 theless in application to a very large range of characters, 

 I am satisfied that the distinction holds, and that in 

 analysis it is a useful aid. Now we have no difficulty 

 in finding evidence of variation by loss. Examples 

 abound, but variations by addition are rarities, even if 

 there are any which must be so accounted. The 

 \ariations to which inter-specific sterility is due are 

 obviously variations in which something is apparently 

 added to the stock of ingredients. It is one of the 

 common experiences of the breeder that when a hybrid 

 is partially sterile, and from it any fertile offspring can 

 be obtained, the sterility, once lost, disappears. This 

 has been the history of many, perhaps most of our 

 cultivated plants of hybrid origin. 



The production of an indubitably sterile hybrid from 

 completely fertile parents which have arisen under 

 critical observation from a single common origin is the 

 event for which we wait. Until this event is witnessed, 

 our knowledge of evolution is incomplete in a vital 

 respect. From time to time a record of such an obser- 

 \ ation is published, but none has yet survived criticism. 

 Meanwhile, though our faith in evolution stands un- 

 shaken, we have no acceptable account of the origin of 

 " species." 



Curiously enough, it is at the same point that the 

 validity of the claim of natural selection as the main 

 directing force was most questionable. The survival 

 of the fittest was a plausible account of evolution in 

 broad outline, but failed in application to specific 

 difference. The Darwinian philosophy convinced us 

 that every species must " make good " in nature if it is 

 to survive, but no one could tell how the differences — 

 often very sharply fixed — which we recognise as 

 specific, do in fact enable the species to make good. 

 The claims of natural selection as the chief factor in the 

 determination of species have consequently been dis- 

 credited. 



I pass to another part of the problem, where again, 

 though extraordinary progress in knowledge has been 

 made, a new and formidable difficulty has been en- 

 countered. Of variations we know a great deal more 

 tlian we did. Almost all that we have seen are varia- 



' He refers to it, however, in " Animals and Plants," chap, xix., and 



i'luces the sterility which he observed in several of his illegitimately 



;^ed plants of Ly thrum, argtiing that this sterility, arising from the crossing 



I o-derivatives, is comparable with that produced by the intercrossing 



true species. The details ore given in " Forms of Flowers," chap. v. 



\\ ilhout more evidence the genetical nature of these plants cannot be 



• onjectured with much confidence, but it is highly improbable that the 



jMrallcl really holds. 



tions in which we recognize that elements have been 

 lost. In addressing the British Association in 1914 I 

 dwelt on evidence of this class. The developments of 

 the last seven years, which are memorable as having 

 provided in regard to one animal, the fly Drosophila, 

 the most comprehensive mass of genetic observation yet 

 collected, serve rather to emphasise than to weaken the 

 considerations to which I then referred. Even in Droso- 

 phila, where hundreds of genetically distinct factors 

 have been identified, very few new dominants, that is to 

 say positive additions, have been seen, and I am assured 

 that none of them are of a class which could be expected 

 to be viable under natural conditions. 



If we try to trace back the origin of our domesticated 

 animals and plants, we can scarcely ever point to a 

 single wild species as the probable progenitor. Almost 

 every naturalist who has dealt with these questions in 

 recent years has had recourse to theories of multiple 

 origin, because our modern races have positive char- 

 acteristics which we cannot find in any existing species, 

 and which combinations of the existing species seem 

 unable to provide. To produce our domesticated races 

 it seems that ingredients must have been added. To 

 invoke the hypothetical existence of lost species pro- 

 vides a poor escape from this difficulty, and we are left 

 with the conviction that some part of the chain of 

 reasoning is missing. The weight of this objection will 

 be most felt by those who have most experience in 

 practical breeding. I cannot, for instance, imagine 

 a round seed being formed on a wrinkled variety of pea 

 except by crossing. Such seeds, which look round, 

 sometimes appear, but this is a superficial appearance, 

 and either these seeds are seen to have the starch of 

 wrinkled seeds or can be proved to be the produce of 

 stray pollen. Nor can I imagine a fern-leaved Primula 

 producing a palm-leaf, or a star-shaped flower producing 

 the old type of sinensis flower. And so on through long 

 series of forms which we have watched for twenty years. 



Analysis has revealed hosts of transferable characters. 

 Their combinations suffice to supply in abundance 

 series of types which might pass for new species, and 

 certainly would be so classed if they were met with in 

 nature. Yet, critically tested, we find that they are not 

 distinct species, and we have no reason to suppose that 

 any accumulations of characters of the same order 

 would culminate in the production of distinct species. 

 Specific difference therefore must be regarded as pro- 

 bably attaching to the base upon which these trans- 

 ferables are implanted, of which we know absolutely 

 nothing at all. Nothing that we have witnessed in the 

 contemporary world can colourably be interpreted as 

 providing the sort of evidence required. 



Twenty years ago, de Vries made what looked like a 

 promising attempt to supply this so far as Oenothera 

 was concerned. In the light of modern experiments, 

 especially those of Renner, the interest attaching to the 

 polymorphism of Oenothera has greatly developed, but 

 in application to that phenomenon the theory of muta- 

 tion falls. We see novel forms appearing, but they are 

 no new species of Oenothera, nor are the parents which 

 produce them pure or homozygous forms. Renner's 

 identification of the several complexes allocated to the 

 male and female sides of the several types is a wonderful 

 and significant piece of analysis introducing us to new 

 genetical conceptions. The Oenotheras illustrate in 



NO. 2739, VOL. 109] 



