486 



NATURE 



[September 13, 1900 



wrongly interpreted, and two or three times the amount of time 

 we have demanded may have been consumed in their formation. 

 This is a very obvious possibility, yet again our estimate con- 

 cerning these rocks may be correct, but we may have erroneously 

 omitted to take into account certain portions of the Archrean com- 

 plex, which may represent primitive sedimentary rocks, formed 

 under exceptional conditions, and subsequently transformed 

 under the influence of the internal heat of the earth. This, I 

 think, would be Prof. Bonney's view. Finally Lord Kelvin 

 has argued that the life of the sun as a luminous star is even 

 more briefly limited than that of our oceans. In such a case if 

 our oceans were formed fifty-five millions of years ago, it is 

 possible that after a short existence as almost boiling water they 

 grew colder and colder, till they became covered with thick ice, 

 and moved only in obedience to the tides. The earth, frozen 

 and dark, except for the red glow of her volcanoes, waited the 

 coming of the sun, and it was not till his growing splendour had 

 banished the long night that the cheerful sound of running 

 waters was heard again in our midst. Then the work of 

 denudation and deposition seriously recommenced, not to cease 

 till the life of the sun is spent. Thus the thickness of the 

 stratified series may be a measure rather of the duration of sun- 

 light than of the period which has elapsed since the first forma- 

 tion of the ocean. It may have been so — we cannot tell — but 

 it may be fairly urged that we know less of the origin, history, 

 and constitution of the sun than of the earth itself, and that, for 

 aught we can say to the contrary, the sun may have been shining 

 on the just-formed ocean as cheerfully as he shines to-day. 



Time required for the Evolution of the Living World. 



But, it will be asked, how far does a period of twenty-six 

 millions satisfy the demands of biology? Speaking only for 

 myself, although I am aware that eminent biologists are not 

 wanting who share this opinion, I answer, Amply. But it will 

 be exclaimed. Surely there are " comparisons in things." Look 

 at Egypt, where more than 4,000 years since the same species 

 of man and animals lived and flourished as to-day. Examine 

 the frescoes and study the living procession of familiar forms 

 they so faithfully portray, and then tell us, how comes it about 

 that from changes so slow as to be inappreciable in the lapse of 

 forty centuries you propose to build up the whole organic world 

 in the course of a mere twenty-six millions of years? To all 

 which we might reply that even changeless Egypt presents us 

 with at least one change — the features of the ruling race are 

 to-day not quite the same as those of the Pharaohs. But 

 putting this on one side, the admitted constancy in some few 

 common forms proves very little, for so long as the environment 

 remains the same natural selection will conserve the type, and, 

 so far as we are able to judge, conditions in Egypt have 

 remained remarkably constant for a long period. 



Change the conditions, and the resulting modification of the 

 species becomes manifest enough ; and in this connection it is 

 only necessary to recall the remarkable mutations observed and 

 recorded by Prof. Weldon in the case of the crabs in Plymouth 

 Harbour. In response to increasing turbidity of the sea water 

 these crabs have undergone or are undergoing a change in the 

 relative dimensions of the carapace, which is persistent, in one 

 direction, and rapid enough to be determined by measurements 

 made at intervals of a few years. 



Again, animals do not all change their characters at the same 

 rate : some are stable, in spite of changing conditions, and these 

 have been cited to prove that none of the periods we look upon 

 as probable, not twenty-five, not a hundred millions of years, 

 scarce any period short of eternity, is sufficient to account for 

 the evolution of the living world. If the little tongue-shell, 

 Lingula, has endured with next to no perceptible change from 

 the Cambrian down to the present day, how long, it is some- 

 times inquired, would it require for the evolution of the rest of 

 the animal kingdom ? The reply is simple: the cases are dis- 

 similar, and the same record which assures us of the persistency 

 of the Lingula tells us in language equally emphatic of the 

 course of evolution which has led from the lower organisms 

 upwards to man. In recent and Pleistocene deposits the relics 

 of man are plentiful : in the latest Pliocene they have dis- 

 appeared, and we encounter the remarkable form Pithecan- 

 thropus ; as we descend into the Tertiary systems the higher 

 mammals are met with, always sinking lower and lower in the 

 scale of organisation as they occur deeper in the series, till in 

 the Mesozoic deposits they have entirely disappeared, and their 

 place is taken by the lower mammals, a feeble folk, offering 



NO. i6ji, vol. 62] 



little promise of the future they were to inherit. Still lower, 

 and even these are gone ; and in the Permian we encounter 

 reptiles and the ancestors of reptiles, probably ancestors of 

 mammals too ; then into the Carboniferous, where we find 

 amphibians, but no true reptiles ; and next into the Devonian, 

 where fish predominate, after making their earliest appearance 

 at the* close of the Silurian times ; thence downwards, and the 

 vertebrata are no more found — we trace the evolution of the 

 invertebrata alone. Thus the orderly procession of organic 

 forms follows in precisely the true phylogenetic sequence ; in- 

 vertebrata first, then vertebrates, at first fish, then amphibia, 

 next reptiles, soon after mammals, of the lowlier kinds first, of 

 the higher later, and these in increasing complexity of structure 

 till we finally arrive at man himself. While the living world 

 was thus unfolding into new and nobler forms, the immutable 

 Lingula simply perpetuated its kind. To select it, or other 

 species equally sluggish, as the sole measure of the rate of bio- 

 logic change would seem as strange a proceeding as to confound 

 the swiftness of a river with the stagnation of the pools that lie 

 beside its banks. It is occasionally objected that the story we 

 have drawn from the palreontological record is mere myth or 

 is founded only on negative evidence. Cavils of this kind prove 

 a double misapprehension, partly as to the facts, partly as to the 

 value of negative evidence, which may be as good in its way as 

 any other kind of evidence. 



Geologists are not unaware of the pitfalls which beset negative 

 evidence, and they do not conclude from the absence of fossils 

 in the rocks which underlie the Cambrian that pre-Cambrian 

 periods were devoid of life ; on the contrary, they are fully 

 persuaded that the seas of those times were teeming with a rich 

 variety of invertebrate forms. How is it that, with the exception 

 of some few species found in beds immediately underlying the 

 Cambrian, these have left behind no vestige of their existence? 

 The explanation does not lie in the nature of the sediments, 

 which are not unfitted for the preservation of fossils, nor in the 

 composition of the then existing sea water, which may have 

 contained quite as much calcium carbonate as occurs in our pre- 

 sent oceans ; and the only plausible supposition would appear 

 to be that the organisms of that time had not passed beyond 

 the stage now represented by the larvae of existing invertebrata, 

 and consequently were either unprovided with skeletons, or at 

 all events with skeletons durable enough for preservation. If 

 so, the history of the earlier stages of the evolution of the in- 

 vertebrata will receive no light from palaeontology ; and no direct 

 answer can be expected to the question whether, eighteen or 

 nineteen millions of years being taken as sufficient for the 

 evolution of the vertebrata, the remaining available eight 

 millions would provide for that of the invertebrate classes 

 which are represented in the lowest Cambrian deposits. On 

 a priori grounds there would appear to be no reason why it should 

 not. If two millions of years afforded time enough for the 

 conversion of fish into amphibians, a similar period should 

 suffice for the evolution of trilobites from annelids, or of anne- 

 lids from trochospheres. The step from gastrulas to trocho- 

 spheres might be accomplished in another two millions, and 

 two millions more would take us from gastrulas through morulas 

 to protozoa. 



As things stand, biologists can have nothing to say either for 

 or against such a conclusion : they are not at present in a posi- 

 tion to offer independent evidence ; nor can they hope to be so 

 until they have vastly extended those promising investigations 

 which they are only now beginning to make into the rate of the 

 variation of species. 



Unexpected Absence of Thermal Metamorphosis in Ancient 

 Rocks. 



Two difficulties now remain for discussion : one based on 

 theories of mountain chains, the other on the unaltered state of 

 some ancient sediments. The latter may be taken first. Prof, 

 van Hise writes as follows regarding the pre-Cambrian rocks of 

 the Lake Superior district : "The Penokee series furnishes an 

 instructive lesson as to the depth to which rocks may be buried 

 and yet remain but slightly aftectea by metamorphosis. The 

 series itself is 14,000 feet thick. It was covered before being 

 upturned with a great thickness of Keweenaw rock.. This 

 series at the Montreal River is estimated to be 50,000 feet thick. 

 Adding to this the known thickness of the Penokee series, we have 

 a thickness of 64,000 feet. . . . The Penokee rocks were then 

 buried to a great depth, the exact amount depending upon their 



