154 



NATURE 



[September 29, 192 1 



most part, to be merely fluctuations, oscillating 

 about a mean, and therefore incapable of giving 

 rise to permanent new types. The well-estab- 

 lished varieties of the Darwinian, such as the 

 countless forms of Erophila verna, are now inter- 

 preted as elementary species, no less stable than 

 Linnean species, and of equally unknown origin. 

 The mutations of De Vries, though still accepted 

 at their face value by some biologists, are sus- 

 pected by others of being nothing more than 

 Mendelian segregates, the product of previous 

 crossings ; opinion on this subject is in a state of 

 flux. In fact, it is clear that we know astonish- 

 ingly little about variation. Dr. Lotsy, indeed, 

 proposes to dispense with variation altogether, 

 and to find the true origin of species in Mendelian 

 segregation ; inheritable variability, he believes, 

 does not exist ; new species, on his bold hypo- 

 thesis, arise by crossing, and so, as he points out, 

 we may have an evolution, though species remain 

 constant. Thus everything apparently new 

 depends on a re-combination of factors already 

 present in the parents. "The cause of evolution 

 lies in the interaction of two gametes of different 

 constitution." 



I am aware that very surprising results have 

 been obtained by crossing. Nothing could well 

 have been more striking than the series of 

 Antirrhinum segregates which Dr. Lotsy showed 

 us some years ago at a meeting of the Linnean 

 Society ; and now we hear of an apetalous Lychnis 

 produced by the crossing of normally petaloid 

 races. We do not know yet to what extent that 

 sort of thing goes on in Nature, or what chance 

 such segregates have of surviving. Still, if one 

 may judge by Dr. Lotsy 's experimental results, 

 ample material for natural selection to work on 

 might be provided in this way. 



Dr. Lotsy 's theory that new species originate 

 by Mendelian segregation, if true, would have the 

 advantage that it w^ould make quite plain the 

 meaning of sexual reproduction. Hitherto there 

 has been a good deal of doubt ; some authorities 

 have held that sexual reproduction stimulated, 

 others that it checked, variation. But, if we 

 eliminate variation, and rely solely on the pro- 

 ducts of crossing, we get a clear view — " species, 

 as well as individuals, have two parents "; sexual 

 reproduction can alone provide adequate material 

 for new forms, and can provide it in unbounded 

 variety. 



Again, though Dr. Lotsy himself is far from 

 sanguine on this point, the crossing theory might 

 be helpful to the evolutionary morphologist, for 

 breeding is open to unlimited experiment, and we 

 might hope to learn what kinds of change in 

 organisms are to be expected. For example, the 

 Lychnis experiment shows how easily a petaloid 

 race may become apetalous. Such results might 

 ultimately be a great help in unravelling the 

 course of evolution in the past. We should gam 

 an idea of the transforrnations which might 

 actually have taken place, excluding those which 

 were out of the question. At present all specula- 

 tion on the nature of past changes is in the air, , 

 NO. 2709, VOL. 108] 



for variation itself is only an hypothesis, and we 

 have to decide, quite arbitrarily, what kind of 

 variations we think may probably have occurred 

 in the course of descent. One need only recall 

 the various theories of the origin of the seed from 

 the megasporangium to realise how arbitrary such 

 speculations are. 



But, while recognising certain advantages in 

 the theory of the origin of species by crossing, 

 it is not for me to pronounce any opinion 

 as to its truth. It is only the present 

 position of the question that concerns us 

 to-day. Some modern geneticists believe th;it 

 there is evidence for mutation by the loss i^l 

 factors, apart from the effects of crossing. 

 Dr. Lotsy considers that such changes, if proved, 

 can afford no explanation of progressive evolu- 

 tion. " Evolution by a process of repeated losses 

 is inconceivable." It has, however, been pointed 

 out by Dr. Agnes Arber, in her recent admirable 

 book on water-plants, that, on any theory of 

 evolution, " what organisms have gained in 

 specialisation they have lost in plasticity." This 

 is true, but it is not clear that this admitted loss 

 of potentialities is the same thing as the loss of 

 factors, in the sense of genetics. 



Turning for a moment to Darwin's own theory 

 of the origin of species by means of natural selec- 

 tion, the efficacy of the latter in weeding out the 

 unfit is, of course, still acknowledged, and some 

 geneticists allow it a considerable rdle. But there 

 is a strong tendency in these days to admit natural 

 selection only as a "merely negative force," and 

 as such it has even been dismissed as a " truism." 

 Now Darwin's great book was most certainly not 

 written to enunciate a truism. He regarded 

 natural selection as " the most important, but not 

 the exclusive, means of modification " (" Origin of 

 Species," p. 4). It was the continual selection of 

 the more fit, the " preservation of favoured 

 races," on which he relied, and not the mere 

 obvious elimination of the unfit, and this great < 

 idea (so imperfectly understood by many of his 

 contemporaries and successors) he worked out 

 with astonishing power, in the light of the changes 

 which man has produced, with the help of his 

 own artificial selection. 



It may be that the theory of natural selection, 

 as Darwin and Wallace understood it, may some 

 day come into its own again ; certainly it illumi- 

 nated, as no other theory has yet done, the great 

 subject of adaptation, which to some of us is, and 

 remains, the chief interest of biology. But in 

 our present total ignorance of variation and doubt 

 as to other means of change, we can form no clear 

 Idea of the material on which selection has had to 

 work, and we must let the question rest. 



For the moment, at all events, the Darwinian 

 period is past ; we can no longer enjoy the com- 

 fortable assurance, which once satisfied so many 

 of us, that the main problem had been solved — all 

 is again in the melting-pot. By now, in fact, a 

 new generation has grown up that knows not 

 Darwin. 



Yet evolution remains — we cannot get away 



