October 3, 1901] 



NA TURE 



569 



Bateson has measured the horns of the heads of 343 rhitioceros 

 beetles and has got a bimodal polygon. The polygon with the 

 lower mode has a skewness of +0'48; that with the higher 

 mode a skewness of -O'oj. One might infer that the right- 

 hand form, the long-horned beetles, had diverged less than the 

 short-horned from the ancestral condition. -Again, as is well- 

 known, the chinch bug occurs in two forms — the long-winged 

 and the short-winged. Now, in a forthcoming paper my pupil, 

 Mr. Garber, will show that the frequency polygon of the short- 

 winged form has a skewness of -fO'44, while that of the long- 

 winged form has a skewness of - o'43. On our fundamental 

 hypothesis the ancestral condition must have been midway 

 between the modes. 



Still a third class of cases that gives evidence as to the sig- 

 nificance of skewness is that where two place modes have moved 

 in the same direction but in different degrees. Thus the index 

 (breadth -f length) of the shell of Littorina litloren, the shore 

 snail, as measured by Bumpus has, at Newport, a mode of 90 ; 

 at Casco Bay, of 93. The skewness is positive in both places 

 and greater ( + '24) at the more southern point than at Casco 

 Bay (4- ■13). This indicates that the ancestral races had a 

 higher index even than those of Casco Bay, probably not far 

 from 96, and also that the Littorina littorca of our coast came 

 from the northward, since the northern shells are the rounder. 

 We have historical evidence that they did come from the north- 

 ward. Likewise the Littorinas from South Kincardineshire, 

 Scotland, have a modal index of 88 and a skewness of -foo55, 

 while those of the Humber, with a mode of 91 have a skewness 

 of H-0'048. These figures suggest that if the mode were 97 the 

 skewness would be o, and this svould give practically the same 

 value to the ancestral index as arrived at for the Littorinas of 

 our coast. It will be seen from these illustrations that the 

 form of the frequency polygon may be of use in determining 

 phylogeny. 



While skewness is thus often reminiscent, we must not forget 

 the possibility that it may be, in certain cases, prophetic. This 

 has come out rather strongly in a piece of work I have been 

 engaged on during the past year. I have been counting the 

 number of rays in recent Pectin irradians from various localities, 

 and have obtained in some cases evident skewness in the fre- 

 quency polygons. To see what phylogenetic meaning, if any, 

 this skewness has I sought to get a series of late fossils. After 

 careful consideration I was led to go to the Nansemond River 

 for the late Tertiary fossils found there and already referred to ; 

 these served my purpose admirably. We may now compare the 

 average number of rays from the two extreme layers at Jack's 

 Bank and at Morehead with the indices of skewness of the 

 frequency polygons from the same localities. 



Place. Av. No. of rays. Index of skewness <r 



Morehead, N.C I7'3 ... -009 ... o'8l 



Upper layer, Jack'sBank ... 217 ... -o'i6 ... fio 



Lower ,, ,, ,, ... 22'6 ... -o'22 ... i '24 



This series is instructive in that it tells us that the gradual 

 reduction in number of rays has been accompanied at each pre- 

 ceding stage by a negative skewness. This skewness was thus 

 prophetic of what was to be. The skew condition of the fre- 

 quency polygon we may attribute to a selection taking place at 

 every stage, and the interesting result appears that the selection 

 diminishes in intensity from the earliest stage onward. It is as 

 though perfect adjustment were being acquired. If adjustment 

 were being perfected we might expect a decrease in the varia- 

 bility in the rays at successive periods. And we do find such 

 a decrease. This is indicated in the last column, where a 

 stands for the index of variability. From this column it ap- 

 pears that the variation in the number of rays has diminished 

 from I '24 rays in the Miocene to o'8i ray in recent times. 

 This fact again points to an approach to perfection and stability 

 on the part of the rays. Exactly why or wherein the reduced 

 number of rays is advantageous I shall not pretend to say. It 

 is quite possible that it is not more advantageous, but that there 

 is in the phylogeny of Peiten irradians an inherent tendency 

 towards a reduction in the number of multiple parts. As a 

 matter of fact there are other Pectens in which the number of 

 rays is less even than in irradians. 



The reduction in the variability of the rays with successive 

 geological periods has another interest in view of the theory of 

 Williams and of Rosa, according to which evolution and differ- 

 entiation have of necessity been accompanied by a reduction in 

 variability. Evolution consists, indeed, of a splitting off of the 



NO. 1666, VOL. 64] 



extremes of the range of variation, so that in place of species 

 with a wide range of variability we have two or three species 

 each with a slight range of variability. In the particular case 

 in hand, however, it is not certain that the lower Jack's Bank 

 form-unit (named Peclen eboretis by someone) has given rise to 

 any other form than something of which Peclen "irradians" 

 of Morehead is a near representative. The evidence indicates 

 that the reduced variability is solely the effect of the skewing 

 factors. 



The upshot of this whole investigation into the biological 

 significance of skew variation is, then, this : .Skewness is sometimes 

 reminiscent and sometimes prophetic. In our present state of 

 knowledge it is not possible by inspecting a single skew curve to 

 say which of the two interpretations is correct in the given case. 

 But by a comparison of the frequency curves of allied form-units 

 the state of affairs can usually, as in the examples given, be 

 inferred. A method of interpreting the single skew curve is a 

 discovery for the future. 



■ I realise that I have been bold, not to say rash, in this attempt 

 to foreca.st the zoology of the twentieth century. I suppose, 

 after all, I have merely expressed my personal ideals. Let 

 those comfort themselves, therefore, who like my picture not 

 and let them draw one more to their taste. These matters of 

 detail are after all less important, but the general trend of the 

 science I believe to be determined by the great general laws that 

 will hold, whatever the detailed lines of development. First, 

 students of the science will cling closer to inductive methods 

 without abandoning deduction. Speculative web-spinning will 

 be less common, will be less attractive, and will be more 

 avoided by naturalists of repute. Great generalisations will be 

 made, of course, but made with caution and founded at every 

 step on facts. Second, the science will deal more with processes 

 and less with static phenomena ; more with causes and less with 

 the accumulation of data. The time is coming when the 

 naturalist who merely describes what he sees in his sections will 

 have neither more nor less claim for consideration than he who. 

 describes a new variety of animal. It is relations, not facts, that 

 count. Third, the science will become experimental, at least in . 

 so far as it deals with processes. Nothing will be taken for 

 granted that can be experimentally tested. Better experimental 

 laboratories will be founded and larger experimental stations, 

 such as Bacon foresaw in the new world, will be established; 

 Fourth, the science will become more quantitative. This is the 

 inexorable law of scientific progress, at least where processes 

 are concerned. I repeat that there is no reason to expect or 

 desire the abandoning of the lines of work already recognised 

 and followed for a half century or more. Rather, holding fast 

 to and extending the old lines of investigation, zoology will be 

 enriched by new fields of study lying between and uniting the 

 old. As chemistry and physics are uniting and occupying the 

 intervening field, as geology and botany are coming close 

 together in plant ecology, so will zoology and mathematics, 

 zoology and geology, zoology and botany find untouched fields 

 between them and common to them. Working in these new 

 fields and by the aid of new methods the naturalist of the 

 future will penetrate further into the nature of processes and 

 unravel their causes. 



The zoology of the twentieth century will be what the 

 zoologist of the twentieth century makes it. One hundred 

 years ago the prerequisites of the naturalists were few, and the 

 opportunities of getting them were small. He must have 

 studied with some master or have worked as an assistant under 

 a naturalist in some museum. The places were few, the masters 

 often difficult of approach. Now, while on the one hand the 

 training required is vastly more exacting, on the other hand 

 the opportunities are generous. Just because of the fact that 

 zoology is spreading to and overlapping the adjacent sciences, . 

 the zoologist must have his training broadened and lengthened. 

 .\ zoologist may well be expected to know the modern languages 

 (let us hope this requirement may not be further extended), 

 mathematics through analytics, laboratory methods in organic 

 as well as inorganic chemistry, the use of the ordinary physical 

 instruments, advanced geology and physiography, botany, 

 especially in its ecological, physiological and cytological aspects, 

 and animal palceontology. The list of prerequisites is appal, 

 lingly long ; zoologists of the future will be forced to an earlier 

 and narrower specialisation, while at the same time they must 

 lay a broader foundation for it. 



But if the prerequisites of the zoologists are to be numerous, 

 their acquisition will be easy. Even now scores of universities 



