September 24, 1896] 



NA TURE 



501 



whereas, "if the mathematicians are ri(»ht, the biologists cannot 

 have what they demand. . . . The jelly-fish would have been 

 dissipate'! in steam long before he had had a chance of display- 

 ing the advantageous variation which was to make him the 

 ancestor of the human race." 



The second objection was that " we cannot demonstrate the 

 process of natural selection in detail ; we cannot even, with 

 more or less ease, imagine it." '• In natural selection who is to 

 supply the breeder's place?'' ''There would be nothing but 

 mere chance to secure that the advantageously varied bridegroom 

 at one end of the wood should meet the bride, who by a happy 

 contingency had been advantageously varied in the same direction 

 at the s.inie time at the other end of the wood. It would be a 

 mere ehance if they ever knew of each other's existence — a still 

 more unlikely chance that they should resist on both sides all 

 temptaiions to a less advantageous alliance. But unless they 

 <lid so the new breed would never even begin, let alone the 

 question of its perpetuation after it had begun." 



I'rof. Huxley, in seconding the vote of thanks to the 

 rresidenl, said that he could imagine that certain parts of the 

 address might raise a very good discussion in one of the Sections, 

 and I have little doubt that he referred to these criticisms and to 

 this .'"eciion. When I had to face the duty of preparing this 

 address. I could find no subjects better than those provided by 

 Lord Salisburj'. 



At first the second objection seemed to offer the more attrac- 

 tive suliject. It was clear that the theory of natural selection as 

 held by Darwin was misconceived by the speaker, and that the 

 criticism was ill-aimed. Darwin and Wallace, from the verj' 

 first, considered that the minute differences which separate 

 individuals were of far more importance than the large single 

 variations which occasionally arise — Lord Salisbury's advan- 

 tageously varied bride and bridegroom at opposite ends of the 

 wood. In fact, after Fleeming Jenkins's criticisms in the 

 N'orth Piilish A'efie-u for June 1S67. Darwin abandoned these 

 large single variations altogether. Thus he wrote in a letter to 

 ■Wallace February 2, 1S69) : "I always thought individual 

 differences more important ; but I was blind, and thought single 

 variations might be preserved much oftener than I now see is 

 possible or probable. I mentioned this in my former note 

 merely because I believed that you had come to a similar con- 

 clusion, and I like much to be in accord with you." ("Life 

 and Letters," vol. iii.) Hence we inay infer that the other 

 great discoverer of natural selection had come to the same con- 

 clusion at an even earlier date. But this fact removes the whole 

 point from the criticism I have just quoted. According to the 

 Darwin-Wallace theory of natural selection, individuals 

 sufficiently advantageously varieil to become the material for a 

 fresh advance when an advance became necessary, and at other 

 times sutiicient to maintain the ground previously gained — such 

 individuals existed not only at the opposite ends of the wood, 

 but were common enough in every colony within its confines. 

 The mere fact that an individual had been able to reach the con- 

 dition of a possible bride or bridegroom would count for much. 

 Few will dispute that such individuals " have already success- 

 fully run the gauntlet of by far the greatest dangers which beset 

 the higher animals [and, it may be added, the lower animals also] 

 — the dangers of youth. Natural selection has already pro- 

 nounced a satisfactory verdict upon the vast majority of animals 

 which have reached maturity." (Poulton, "Colours of 

 Animals," p. 308.) 



But the criticism retains much force when applied to another 

 theory of evolution by the selection of large and conspicuous 

 variations, a theory which certain writers have all along sought 

 to add to or substitute for that of Darwin. Thus Huxley from 

 the very first considered that Darwin had burdened himself 

 unnecessarily in rejecting /fr .wZ/ww evolution so unreservedly. 

 (See his letter to Darwin, November 23, 1859; "Life and 

 Letters," vol. ii. ) And recently this view has been revived by 

 Bateson's work on variation and by the writings of Francis 

 (iaiton. I had at first intended to attempt a discussion of this 

 view, together with Lord Salisbury's and other objections which 

 may be urged against it ; but the more the two were considered, 

 the more pressing became the claims of the criticism alluded to 

 at first— the argument that the history of our planet does not 

 allow sufficient time for a process which all its advocates admit 

 to be extremely slow in its operation. I select this subject 

 because of its transcendent importance in relation Xcr organic 

 evolution, and because I hope to show that the naturalist 

 has something of weight to contribute to the controversy which 



NO. 1404, VOL. 54] 



has been waged intermittently ever since Lord Kelvin's 

 paper "On Geological Time"' appeared in 1868. It has 

 been urged by the great worker and teaclier who occupied the 

 Presidential Chair of this Association when it last met in this city 

 that biologists have no right to take part in this discussion. In 

 his Anniversary Address to the Cieological Society in 1869 

 Huxley .said : " Biology takes her time from geology. ... If 

 the geological clock is wrong, all the naturalist will have to do 

 is to modify his notions of the rapidity of change accordingly." 

 This contention is obviously true as regards the time which has 

 elapsed since the earliest fossiliferous rocks were laid down. 

 For the duration of the three great periods we must look to the 

 geologist ; but the question as to whether the whole of organic 

 evolution is comprised within these limits, or, if not, what pro- 

 portion of it is so contained, is a question for the naturalist. 

 The naturalist alone can tell the geologist whether his estimate 

 is sufficient, or whether it must be multiplied by a small or by 

 some unknown but certainly high figure, in order to account for 

 the evolution of the earliest forms of life known in the rocks. 

 This, I submit, is a most important contribution to the dis- 

 cussion. 



Before proceeding further it is right to point out that obviously 

 these arguments will have no weight with those who do not 

 beliexe that evolution is a reality. But although the causes of 

 evolution are greatly debated, it may be assumed that there is no 

 perceptible difference of opinion as to evolution itself, and this 

 common ground will bear the weight of all the zoological argu- 

 ments we shall consider to-day. 



It will be of interest to consider first how the matter pre- 

 sented itself to naturalists before the beginning of this controversy 

 on the age of the habitable earth. I will content myself with 

 quotations from three great writers on biological problems— men 

 of extremely different types of mind, who yet agreed in their 

 conclusions on this subject. 



In the original edition of the "Origin of Species" (1859), 

 Darwin, arguing from the presence of trilobites, Nautilus, 

 Lingula, &c., in the earliest fossiliferous rocks, comes to the 

 following conclusion (pages 306, 307): " Consequently, if my 

 theory be true, it is indisputable that before the lowest Silurian 

 stratum was deposited long |jeriods elapsed, as long as, or pro- 

 bably far longer than, the whole interval from the Silurian age 

 to the present day ; and that during these vast yet quite 

 unknown periods of time the world swarmed with living 

 creatures." 



The depth of his conviction in the validity of this conclusion is 

 seen in the fact that the passage remains substantially the same 

 in later editions, in which, however, Cambrian is substituted for 

 Silurian, while the words " yet quite unknown " are omitted, as 

 a concession, no doubt, to Lord Kelvin's calculations, which he 

 then proceeds to discuss, .admitting as possible a more rapid 

 change in organic life, induced h)y more violent physical changes. 

 (Sixth ed., 1872, p. 286.) 



We know, however, that such concessions troubled him much, 

 and that he was really giving U]i what his judgment still 

 approved. Thus he wrote to Wallace on April 14, 1869: 

 " "Thomson's views of the recent age of the world have been for 

 some time f)ne of my sorest troubles. . . ." And again, on 

 July 12, t87i, alluding to Mivart's criticisms, he says : " I can 

 say nothing more about missing links than what I have said. I 

 should rely much on pre-Silurian times ; but then comes Sir W. 

 Thomson, like an odious spectre." 



Huxley's demands for time in order to account for pre- 

 Cambrian evolution, as he conceived it, were far more extensive. 

 .\lthough in 1869 he bade the naturalist stand aside and take no 

 part in the controversy, he had nevertheless spoken as a natur- 

 alist in 1862, when, at the close of another Anniversary Address 

 to the same Society, he argued from the prevalence of persistent 

 types "that any admissible hypothesis of progressive modification 

 must be compatible with persistence without progression through 

 indefinite periods" ; and then maintained that "should such an 

 hypothesis eventually be proved to be true . . . the conclusion 

 will inevitably |iresent itself that the Paleozoic, Mesozoic, and 

 Cainozoic fauna- and florae, taken together, bear somewhat the 

 same proportion to the whole series of living beings which have 

 occupied this globe as the existing fauna and flora do to them." 



1 i rails. Ccol. Soc, C'.lasgow, vol. iii. See also " On the Age of the 

 Sun's Heat," Maciiiitlait.^' March 1862 : reprinted as Appendix to Thomson 

 and Tait, " Natural Philosophy," vol. i. part 2, second edition ; and " On 

 the Secular Cooling of the Earth," Royal Society of Edinburgh, t862. 



