June sS, 1894] 



NA TURE 



197 



wild horses of Circassia, which are greatly exposed to attacks of 

 ■wolves and to extreme vicissitudes of climate, swiftness, strength, 

 wariness, and a hardy constitution must be kept at a high level 

 of efficiency liy the elimination of the less gifted in these 

 qualities ; so that here again the birth-mean must be below the 

 survival-mean. In such cases as these there seems no difficulty 

 in the fact that the mean characters do not change for many 

 generations ; for this is in accordance with Darwin's principle 

 that natural selection " cannot produce absolute perfection, but 

 only relative perfection." When the average characters of a 

 species have reached a point such that it can permanently main- 

 tain itself in a i>iven area, then no further change will occur; 

 but, the less efTicient being constantly weeded out, the survival- 

 mean wiii be necessarily a litlle above the birth-mean. Both 

 tneans will, however, be sensibly permanent as long as the en- 

 vironment remains unchanged. 



Mr. Weldon says that it has not been shown that, in some 

 given case. Panmixia does in fact occur ; and further, that in 

 the only case which has been experimentally investigated — that 

 of the stature of civilised Englishmen — the consequences said to 

 result from it do not, in fact, occur. To obtain absolute evi- 

 dence of Panmixia, or of the action of Natural Selection, is 

 extremely difficult, because we cannot first compare and measure 

 minutely a large number of individuals in a state of nature, and 

 then follow those same individuals throughout their lives and 

 see how nature deals wiih them. We can, however, observe 

 what happens in the case of semi-wild animals, and the 

 examples already cited show that natural selection must, and 

 actually does, act on the character of colour, weeding out those 

 which diverge on both sides towards whiteness or iilackness, 

 and in the case of physical and mental activities destroying 

 those which fall below the standard of excellence requisite for 

 the preservation and continuance of life. 



In our domesticated animals, on the other hand, we find what 

 are probably examples of the effects of Panmixia. The wing- 

 bones of our pigeons, fowls, and ducks, as compared with wild 

 individuals, were found by Darwin to be decidedly reduced in 

 size in proportion to the leg bones ; but a part of this may be 

 due to disuse in the individual, and to determine the share of 

 the two causes seems impos>ible. There are, however, a few 

 characters in which we see Panmixia alone at work in our 

 domesticated animals. Such are, for example, the constant 

 appearance and increase among them of prominent unsymmetrical 

 markings, as in dogs, cats, cattle, and horses. Such markings 

 never occur in wild races, or if they occur in individual cases 

 they never increase; and I have given reasons for thinking that 

 symmetrical colour and marking is kept up in nature for 

 facility of recognition, a factor essential to preservation, 

 and to the formation of new species. In this case, 

 there can be no question of disuse, while as we know that 

 white and unsymmetrical individuals do occasionally occur in 

 wild species, but never increase, the fact of their increase under 

 domestication must be due to the absence of whatever form of 

 natural selection eliminates them in nature ; that is, to Pan- 

 mixia. .-Knother illustration may perhaps be found in the fact 

 of curled tails appearing in domestic pigs and some races of 

 dogs, while no wild animal is known which has a curled tail. 

 We can hardly doubt that the special form of tail in each animal 

 lis of use to it, and that any abnormality, like a curled tail, 

 would be eliminated under nature. Its appearance and per- 

 petuation under domestication is therefore a fair example of 

 Panmixia. 



The slow increase of the stature of civilised Englishmen, 

 jwhich Mr. Gallon is said to have proved, may, it teems to me, 

 'be partly a result of Panmixia, and partly due to more healthy 

 jcondilions of life acting on the individual. It is, I presume, a 

 fact, as generally stated, that old armour shows that the knights 

 of the middle ages were rather short men. This may have been 

 a result of natural selection, because, as a rule, the strongest 

 and most active men are rather under than over middle height ; 

 iwhile tall men would certainly be more exposed to danger, 

 would have to carry a greater weight of armour, and by thus 

 overloading their horses would be under a disadvantage in 

 battle. Tall men would thus be killed off rather faster than 

 short men ; and the same might be the case even after the dis- 

 use of .irmour, so long as rapine and civil war prevailed over a 

 large part of the country. Hut during the last two centuries 

 of comparative peace tall men have been under no such dis- 

 advantage, and their survival may have aided in bringing about 

 the slight increase of average stature which has been observed. 



NO. I 2S7. VOL. 50] 



One other point in Mr. Weldon's communication requires 

 notice. He considers that the frequent occurrence of abnor- 

 malities and the wide range of variation in many species, show 

 that "natural selection is in most cases an imperfect 

 agent in the adjustment of organisms." This conclusion 

 does not appear to me to be a logical one, since 

 it ignores the admitted fact of the exceedingly intermittent 

 character of selection and its constantly varied /t;«/j of aciion. 

 Each species of animal is subject to a number of quite distinct 

 dangers — hunger, cold, wet, disease, and varied enemies — and 

 all these are separately intermittent in their action. Some affect 

 the species at one time of the year only, some at another ; but 

 most of them only reach their maximum of intensity at long 

 intervals — once or twice, perhaps, in a century. Whether cold 

 winters or hot summers, excessive drought or excessive wet, deep 

 snow or phenomenal hail or wind-storms, all are intermittent 

 and occur with extreme severity only at long intervals. These 

 intermittent waves of meteorological phenomena have their 

 corresponding "waves of life," as Mr. Hudson well terms 

 them, such as phenomenal swarms of locusts or of wasps, of 

 caterpillars, mice, or lemmings, and to a less conspicuous degree 

 of almost every living thing. It follows, that during a succes- 

 sion of favourable seasons variation can go on almost unchecked, 

 and even hurtful abnormalities and imperfections may survive 

 for a few years, but soon there comes a check to the increase, 

 and the most abnormal forms die nut ; while after a greater or 

 less interval either adverse seasons or an increase of living 

 enemies weed out all the extreme disadvantageous variations, 

 leaving only the pick of the typical form to continue the race. 

 This may occur again and again, each special period of stress 

 affecting different organs or faculties — now abnormal colour, 

 now deficient agility, now again incaution or a weak digestion 

 — till in turn every departure from the best adapted mean form 

 is eliminated, to again arise and again be extinguished as 

 favourable or unfavourable conditions prevail. Thus, I am fully 

 in agreement with Mr. Thisellon Dyer when he said: "I feel 

 more and more that natural selection is a very hard taskmaster, 

 and that it is down very sharply on structural details that cannot 

 give an account of themselves." (Nature, vol. xxxix. p. 9.) 

 The appearance of imperfect adjustment is thus only a temporary 

 phenomenon, while that there is an underlying permanent adjust- 

 ment is indicated by the long-coniinued identity of specific 

 characters to which Mr. Weldon refers. 



As it is very important to obtain some direct evidence of the 

 action of natural selection, I wish to suggest a mode of doing 

 so which might probably be successful. There is much evidence 

 to show that the migrating birds which visit us in early summer 

 are very largely old birds which have lived through two or more 

 migrations ; and, consetpienlly, that of the large number of 

 young birds which migrate in autumn for the first time a very 

 small proportion return to our shores. If this is so, then the 

 extreme severity of the selection during migration would afford 

 us the opportunity of determining some of the physical characters 

 which influence it, combined no doubt with mental character- 

 istics which we have no means of gauging. I would suggest, 

 therefore, that two or three common species of migrants should 

 be chosen, of which the young birds of the year can be dis- 

 tinguished with certainty. Of these birds a number of observers 

 should collect specimens just before their autumnal migration, 

 and should carefully record the characters fixed upon in the case 

 of the young and old birds separately. Probably the weight, the 

 total length, and the length of the wing, would be sufficient, 

 since heavy birds with comparatively short wings would hardly 

 be adapted for long-continued flight. By laying down the 

 dimensions of some hundreds of specimens in curves of varia- 

 tion, whatever difference existed between the young and old 

 birds would be easily detected ; and this difterence would pre- 

 sumably be the difterence between the birth-mean and the sur- 

 vival-mean, so far as the selective influence of migration is 

 concerned. In the following spring another set of specimens 

 of the same species should be collected and measured ; and we 

 should then perhaps be able to determine the characters which 

 h.ad led to the selection of the young birds which had suivived 

 the double migration. Alfred R. Wallace. 



Discontinuous Colour-Variation. 



I iiAvi; just received a copy of Mr. Bateson's most valuable 

 work on the " Study of Variation " ; and although it will take 

 many weeks to read it as it deserves to be read, a few remarks 



