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analysis of the probabilities in favour of and against the theory that the concurrence of apparent motion was merely accidental, I came to the conclusion that the five large stars and the two smaller ones form a true drifting set. I found, on a moderate computation, that the odds were upwards of half a million to one against the concurrence being accidental; and since I had recognised other instances of concurrence not less striking, I felt that it was morally certain that these stars belong to one star-family.

The reader will perhaps not be surprised to learn, however, that before publishing this conclusion I submitted it (in July, 1869) to one who was, of all men, the best able to pronounce upon its significance the late Sir John Herschel. I have the letter (dated August 1, 1869) which he sent in reply before me as I write. The part relating to my discovery runs as follows:-"The considerations you adduce relative to the proper motions of the stars are exceedingly curious and interesting. Of late years catalogues have gone into much detail, and with such accuracy that these motions are of course much better known to us than some twenty or thirty years ago. The community of proper motion over large regions (of which you give a picture in Gemini and Cancer) is most remarkable, and the coincidence of proper motion in Beta, Gamma, Delta, Epsilon, and Zeta Ursæ Majoris most striking. Your promised paper on this subject cannot fail to be highly interesting."*

In a letter written on May 11, 1870, and referring not to another letter of mine, but to my "Other Worlds," Sir John Herschel remarked, "the cases of star-drift such as that in Ursa Major are very striking, and richly merit further careful examination."

My first public expression of opinion respecting the star-drift in Ursa Major was conveyed in the following terms :-" If these five stars indeed form a system (and I can see no other reasonable explanation of so singular a community of motion), the mind is lost in contemplating the immensity of the periods

He proceeds as follows (the passage is removed from the main text as relating to a different branch of the subject):-"I cannot say that I am at all surprised at its being found that the average proper motions of stars of small magnitudes is not less than those of large, considering (as I have always done) that the range of individual magnitude (i.e. lustre) must be so enormous that multitudes of very minute stars may in fact be our very near neighbours." Compare my paper on "The Sun's Journey through Space," above referred to, which paper also deals with the point touched on in the next sentence of Sir John Herschel's letter:-"Your remark on the conclusion I have been led to draw relative to the small effect of the correction due to the sun's proper motion, will require to be very carefully considered, and I shall of course give it every attention."

which the revolutions of the components of the system must occupy. Mädler had already assigned to the revolution of Alcor around Mizar (Zeta Ursæ) a period of more than 7000 years. But if these stars, which appear so close to the naked eye, have a period of such length, what must be the cyclic periods of stars which cover a range of several degrees upon the heavens." (From Zeta to Beta is a distance on the heavens of about nineteen degrees.) "The peculiarities of the apparent proper motions of the stars," I added, "lend a new interest to the researches which Dr. Huggins is preparing to make into the stellar proper motions of recess and approach."*

But a few months later, in a lecture delivered at the Royal Institution I pointed out more definitely what result I expected from Dr. Huggins's researches. "Before long," I said, "it is likely that the theory of star-drift will be subjected to a crucial test, since spectroscopic analysis affords the means of determining the stellar motions of recess and approach. The task is a very

difficult one, but astronomers have full confidence that in the able hands of Dr. Huggins it will be successfully accomplished. I await the result with full confidence that it will confirm my views." †

It will be manifest that if the five large stars in Ursa are really travelling in the same direction, then, when Dr. Huggins applied the new method of research, he would find that, so far as motion in the line of sight was concerned, these stars were either all receding or all approaching at the same rate, or else that they were all alike in showing no signs of any motion, either of recess or approach.

But in the meantime there was another kind of evidence which the spectroscope might give, and on which I formed some expectations. If these stars form a single system it seemed likely that they would all be found to be constituted alike—in other words, that their spectra would be similar. Not indeed that associated stars always display such similarity. Indeed the primary star of a binary system not unfrequently exhibits a spectrum unlike that of the small companion. But the five large stars in Ursa, being obviously primary members of the scheme they form, might be expected to resemble each other in general constitution. Moreover, since the stars not included in the set-viz., Alpha and Eta-might be regarded as probably very much nearer or very much farther away, it was to be expected (though not so confidently) that these two stars would have spectra unlike the spectrum common (on the supposition) to the five stars.

"Proceedings of the Royal Society," Jan. 20, 1870, pp. 170, 171.

"Report of the Royal Institution of Great Britain," Weekly Evening Meeting, Friday, May 6, 1870, p. 7.

Now, Secchi announced that the stars of the Great Bear, with the exception of Alpha, have spectra belonging to the same type as the spectrum of the bright stars Sirius, Vega, Altair, Regulus, and Rigel. This result was in very pleasing accordance with the anticipations I had formed, except that I should rather have expected to find that the star Eta had a spectrum unlike that of the remaining five stars of the Septentriones. Moreover, as the stars belonging to this particular type are certainly in many cases, and probably in all, very large orbs* (referring here to real magnitude, not to apparent brilliancy), the inference seemed fairly deducible that the drifting five stars are not nearer than Alpha, and therefore (since we have seen that it is unlikely that all the Septentriones lie at nearly the same distance) the inference would be that the drifting stars lie much farther away than the rest.

It remained, however, that the crucial test of motion-measurement should be applied.

In the middle of May last I received a letter from Dr. Huggins announcing that the five are all receding from the earth. In all the hydrogen line called F, is "strong and broad." In the spectrum of Alpha the line F is "not very strong" (so faint, indeed, Dr. Huggins afterwards informed me, that he preferred to determine the star's motion by one of the lines due to magnesium in the star's atmosphere. He found that Alpha is approaching. As to Eta, Dr. Huggins remarked that the line at F is "not so strong or so broad" as in the spectrum of “the five." He was uncertain as to the direction of motion, and mentioned that "the star was to be observed again." He subsequently found that this star is receding. But whereas all the five are receding at the enormous rate of 30 miles per second, Eta's recession was so much smaller that, as we have seen, Dr. Huggins was unable to satisfy himself at a single observation that the star was receding at all.

It will be seen that my anticipations were more than fulfilled. The community of recessional motion was accompanied by evidence which might very well have been wanting-viz., by the discovery that neither Eta nor Alpha shared in the motion. Moreover, the physical association between the five stars was yet further evidenced by the close resemblance found to exist between the spectra of the five stars. Dr. Huggins remarked in his letter, "My expectation had nothing to do with the above results. At the moment, I thought Alpha was included in the

Sirius 4 demonstrably gives out much more light than our sun, and according to the best determinations of his distance he must (if his surface is of equal intrinsic lustre) be from 2,000 to 8,000 times larger than the sun. Vega, Altair, and Rigel are also certainly larger and may be very much larger than our sun.

group, and was therefore a little disappointed when I found Beta going the opposite way."

We have at length, then, evidence, which admits of no question-so obviously conclusive is it to show not only that star-drift is a reality but that subordinate systems exist within the sidereal system. We moreover recognise an unquestionable instance of a characteristic peculiarity of structure in a certain part of the heavens. For, though star-drift exists elsewhere, yet every instance of star-drift is quite distinct in character-the drift in Cancer unlike that in Ursa, and both these drifts unlike the drifts in Taurus, and equally unlike the drift in Aries or Leo. Much more, indeed, is contained in the fact now placed beyond question, than appears on the surface. Rightly understood, it exhibits the sidereal system itself as a scheme utterly unlike what has hitherto been imagined. The vastness of extent, the variety of structure, the complexity of detail, and the amazing vitality, on which I have long insisted, are all implied in that single and, as it were, local feature which I had set as a crucial test of my theories. I cannot but feel a strong hope, then, that those researches which my theories suggest, and which I have advocated during the last few years, will now be undertaken by willing observers. The system of star-gauging, which the Herschels did little more than illustrate (as Sir W. Herschel himself admitted), should be applied with telescopes of different power to the whole heavens,* not to a few telescopic fields. Processes of charting, and especially of equal surface charting, should be multiplied. Fresh determinations of proper motions should be systematically undertaken. All the evidence, in fine, which we have, should be carefully examined, and no efforts should be spared by which new evidence may be acquired. Only when this has been done will the true nature of the galaxy be adequately recognised, its true vastness gauged, its variety and complexity understood, its vitality rendered manifest. To obtain, indeed, an absolutely just estimate of these matters, may not be in man's power to compass; but he can hope to obtain a true relative interpretation of the mysteries of the stellar system. If any astronomer be disposed to question the utility or value of such researches, let him remember that Sir W. Herschel, the greatest of all astronomers, set "a knowledge of the constitution of the heavens," as "the ultimate object of his observations."

This is a work in which telescopes of every order of power would be useful. The observations, also, would be very easily made and would tell amazingly.

391

LIFE-FORMS OF THE PAST AND PRESENT.

BY HENRY WOODWARD, F.G.S., F.Z.S.,

OF THE BRITISH MUSEUM.

[PLATES XC. and XCL]

ONE

NE of the most interesting results at which the naturalist arrives in extending his researches from the living present far back into the remote periods of geological time, is that he finds existing between the life-forms of the past and present, not merely accidental likenesses or analogies, but actual homologies and relationships. There is, in fact, no ground whatever for the old, and to some extent, still prevalent dogma, that the several faunas and floras, which in past ages successively peopled and clothed the surface of the earth, had no direct relationship, either with each other, or with existing types.

Indeed, so strong was this feeling in the minds of nearly all the earlier observers, that they hesitated to compare extinct organisms with living forms, and were content to accept the dictum of those geologists who taught that each series of fossiliferous deposits was a distinct creation, being separated by a universal cataclysm alike from the preceding and subsequent life-periods. It is less than twenty years since the modern doctrine of continuity of life on the earth began to be received and adopted as the basis of all sound paleontological reasoning; and notwithstanding the numerous breaks that still exist, it is nevertheless possible, by reviewing the life-history of any particular class or order, to demonstrate that a real continuity does exist from the earliest representative down to the forms of to-day.

I propose to take, by way of illustration, a few examples from a class which offers perhaps the widest geographical and geological range, combined with the greatest diversity of detail in organisation, of any among the Invertebrate kingdom, namely the Crustacea; confining myself mainly to two of the most ancient orders, the Merostomata and the Trilobita.

In tracing a group like the Crustacea further and yet

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