594 



NA TURE 



[October 20, 1898 



in warmer water and better fed than those in the sea, and 

 therefore on the average were younger after the moult than a 

 number from the sea of the same size. In fact the diminution 

 of frontal breadth depends not on the size of the crab, but on 

 the number of moults it has passed through, whije the size 

 depends on the increase at each moult. A crab which has 

 moulted seven times may be smaller than one which has only 

 moulted five or six times. 



Another case is on record which seems to me to afford an 

 e.Kact parallel to Prof. Weldon's. In Darwin's " Descent of 

 Man " is quoted the evidence of a hunter who asserted that in a 

 certain district male deer with a single unbranched antler were 

 becoming gradually more numerous and taking the place of 

 those with normal branched antlers. The district referred to 

 was that of the Adirondack Hills in North America. As the 

 witness in question had hunted deer for twenty-one years, Darwin 

 considered his evidence important. J. D. Caton, however, who 

 for many years made the Cervidce his special study in Canada, 

 particularly investigated this case. He satisfied himself that 

 there was no truth whatever in the evidence above mentioned. 

 The sHhe-horn bucks seen and killed in the Adirondacks were all 

 yearling bucks with their first antlers. In all species of Cervus 

 the horns which first grow are simple pointed unbranched 

 spikes ; and to prove the existence of spike-horned bucks as a 

 variety, it would be necessary to show that when they cast their 

 horns they developed simple spikes every year throughout life. 

 No attempt was made to prove this, and Caton describes cases 

 which he observed himself, in which spike-horned bucks of 

 unusually large size, which might have been supposed to be full 

 grown, developed branched horns in the following year. 



A final objection to Prof. Weldon's argument may be 

 mentioned. All the crabs on whose measurements he bases the 

 conclusion that the relative frontal breadth of the species in 

 Plymouth Sound has actually decreased within a few years, are 

 small specimens lo to 15 mm., or about half an inch in length 

 of carapace. He makes no attempt to show that the decrease 

 has occurred in adult crabs. The efficiency of filtration would 

 necessarily depend on the absolute size of the filtering 

 mechanism, not on the relative size, since the size of the 

 particles of mud to be excluded remains the same. A crab 

 therefore which survived in consequence of its narrow frontal 

 region at the size of half an inch, would have no advantage 

 when it was 2 or 3 inches long, as the frontal region would then 

 be absolutely much greater. If the mud then kills the small 

 crabs with a broad frontal region, it ought to kill all the adult 

 crabs without exception. 



A simple method of testing the soundness of Prof. Weldon's 

 conclusion, with regard to the crabs in Plymouth Sound, 

 would be to compare the mean frontal breadth of adult crabs 

 from that locality, with that of others collected outside the 

 Sound, e.g. at the mouth of the Yealm, where the water is pure, 

 and at Saltash, where the water is much more turbid. If the 

 sediment in the Sound is really decreasing the mean frontal 

 breadth by a process of selection, that dimension must be 

 greater in clean water, and less where there is still more 

 sediment. J. T. Cunningham. 



Penzance, September 24, 



I SHOULD like to be allowed to make a few remarks upon 

 Prof. Weldon's address to the Zoological Section of the British 

 Association ; for it seems to me — very interesting as it is — that 

 it is entirely outside the real question of the evolution of 

 varieties and species of animals. 



My contention is that individual differences— w\i\\ which Prof. 

 Weldon is solely concerned— a*',? not afford the materials for new 

 varieties or species (I would refer the reader to my paper on 

 "Individual Variations," Natural Science, vol. vi. p. 385). 



A systematist has to consider differences of " form " as well 

 as, and indeed, he regards it as much more important than, 

 "size" or "number." Prof. Weldon, however, refers only 

 to size in crabs and recruits, and to number in pigs' glands and 

 petals of the buttercup. 



If I understand Darwin's theory of " The origin of species 

 by means of natural selection," an individual has some slight 

 variation or new feature, which is beneficial to it in the struggle 

 for life among other organisms, more especially of the same kind 

 —as a " large population " of the same sort is what Darwin 

 and Dr. Wallace demand — then, such an individual may prove 

 itself best fitted to survive and ultimately establish a new 

 variety, the others dying out in the struggle. 



NO. 15 I 2, VOL. 58] 



But, for one or more crabs to have a frontal breadth a little 

 less than that of others in a group of the same kind of crab is 

 no new feature. It is only an "individual difference," such as 

 all organisms are subject to. 



"Number" and "size," to be included in varietal characters 

 must be more pronounced than in the case with the craVjs. The 

 extreme lengths of the carapace are given as 10 i and I4'9 

 millimetres— ?.<;. two- fifths and three-fifths of an inch ; but 

 between those killed by suffocation and the smaller survivors, 

 the greatest difference lies between 8i6-i7 and 787-36, these 

 numbers being the highest "mean frontal breadths in terms of 

 carapace length = 1000"; so that the difference is 28-81, not 

 3 per cent. 



Upon such insignificant differences the life or death of the 

 crabs is supposed to hang ! 



But the contention presumably is that the smaller crabs will 

 form a new variety. Will any zoological systematist accept this ? 



But is it not obvious that if natural selection has been always 

 constantly at work in this supposed way with individual differ- 

 ences among plants and animals, some varieties might be looked 

 for among buttercups and Carcinus vianas ? Take Ranunculus 

 Ficaria, which furnished Mr. Burkill with materials for like 

 observations (Nature, February 7, p. 359, 1895), the petals of 

 which vary much more in number than do those of buttercups. 

 If natural selection has been busy over this species for centuries, 

 how is it that R. Fie. remains R. Fie. still ? for it grows in all 

 sorts of places, favourable and unfavourable. It would be 

 easy to make curves for individual differences for the number 

 of petals, stamens, size of leaves, tubercles, (Sic, but it would 

 all be a waste of energy as far as advancing any illustrations of 

 evolution. Individual differences come up every year, in spite 

 of natural selection and all its imaginary doings. Moreover 

 similar individual differences occur in the leaves all over one 

 and the same tree and of every kind ; what can natural selection 

 do among them ? 



In fact, no one has ever yet shown that a new species has 

 ever arisen out of individual differences " observed in the indi- 

 viduals of the same species inhabiting the same confined area" 

 ("Origin of Species," 6th ed., p. 34). 



The utmost that Prof. Weldon has shown is that, under 

 abnormal and dangerous circumstances, which have killed off 

 other kinds of marine animals from the Sound at Plymouth, 

 crabs are dying out too ; but that the larger ones (older ?) are 

 killed off a little faster, perhaps, than the smaller (Is the orifice 

 to the gills a little larger, so as to allow an easier passage for 

 the mud ?). We may compare this with the London fc^s in 

 winter, which raise the death-rate of older members of the 

 community. 



Prof. Weldon says : " I will show you that in those crabs 

 small changes in size of the frontal breadth do, under certain 

 circumstances, affect the death-rate." 



As this is the very kernel of the whole matter — for he quotes 

 Darwin as saying, "the theory asserts the smallest observable 

 variation " (observe Darwin requires a "variation " ; but there 

 is none at all in any of Prof. Weldon's four examples) "may 

 affect an animal's chance of survival," one anxiously looketl 

 out, on reading the address through, for the fulfilment of this 

 promise ; but near the end, all he says is, that the immediate 

 cause of death was suffocation by mud clogging the gills, and 

 adds : "I think it can be shown that a narrow frontal breadth 

 renders one part of the process of filtration of water more 

 efficient than it is in crabs of greater frontal breadth." 



This opinion is unfortunately no scientific proof; and it is 

 much to be regretted that he did not give us the grounds for 

 his so thinking. 



He only measured the carapaces and frontal breadths, but it is 

 presumable that the legs were proportionally longer in the de- 

 ceased crabs. The question therefore arises, were they, too, 

 concerned in causing the increased death-rate of those with the 

 bigger carapaces? 



Once more, what has all this got to do with evolution ? No 

 one will dispute these interesting illustrations of chance— a 

 name for all cases where one cannot trace actual causes, or 

 inductive evidence — and its application to individual differ- 

 ences ; which, by the way. Dr. Wallace now regards as "non- 

 specific or developmental characters " (Fortnightly Review, 

 March 1895, p. 444), and not leading to new varieties; as he 

 did in his work on " Natural Selection "in 1871. 



Natural selection determines what shall survive and what 

 shall die in the universal struggle for life ; but it has yet to be 

 shown, that the origin of species has anything to do with it. 



