March i, 1888] 



NATURE 



427 



The growth of our knowledge concerning the terrestrial floras 

 and faunas of ancient geological periods, since these words were 

 written in 1869, has constantly forced upon the minds of many 

 geologists the necessity of a duplicate classification of geolc^ical 

 periods, based on the study of marine and terrestrial organisms 

 respectively. 



Upon this important question the judicious remarks of my 

 colleague, Dr. Blanford, must still be fresh in the minds of all 

 geologists and bioloj;ists. lie showed that not only are terres- 

 trial provinces independent of marine ones, but that at the 

 present, as well as in the past, the former are more circumscribed 

 and have an amount of distinctness which does not exist in the 

 case of the latter. 



Nor is it difficult, in the present state of our biological know- 

 ledge, to give a reason for the existence of this state of things. 

 Between completely separated land-areas, migration can only 

 take place by such accidents as the transport of seeds or eggs, or 

 as the consequence of the great but slow changes in the relations 

 of sea and land. Forms adapted only for living in cold climates 

 are isolated by tracts of low-lying tropical land, and, conversely, 

 tropical forms are divided off from one another, by snow-covered 

 mountain-chains, almost as distinctly as by actual oceans. The 

 fact that well-known Arctic plants are found at the top of 

 mountains in tropical or temperate lands, has seemed to many 

 botanists as quite inexplicable without calling in the agency of 

 a general refrigeration, like that which marked the Glacial 

 period. 



But with marine forms of life the case is totally different. 

 The oceans are not only much larger than the continents, but 

 they are all more or less completely connected with one another. 



Forms which live at the surface of the ocean may wander 

 freely in all directions, and know but few limitations except 

 those imposed by temperature, absence of food, &c. ; forms 

 living at moderate depths may migrate along shore-lines or 

 submarine ridges from one area to another ; and even when 

 abysmal tracts of ocean intervene between two litto'-al faunas, 

 recent researches seem to show that the littoral forms of life 

 may wander into such tracts, and eventually, perhaps, cross 

 them, without undergoing extreme or profound modification. 

 In this way, I think, we may account for the important fact so 

 prominently brought into view by Dr. Blanford, that terrestrial 

 life-provinces are and always must have been more restricted 

 in area, and more sharply cut oflf from one another, than marine 

 provinces. 



With the clear recognition of this principle there falls to the 

 ground one of the most frequently urged objections to the uni- 

 formitarian doctrines — that, namely, which is baspd on the sup- 

 posed differences in geographical distribution in ancient times as 

 compared with the present. I have always doubted whether 

 there is any evidence to show that the marine life-provinces of 

 Silurian or Carboniferous times were of greater extent than those 

 of the present day. 



I believe that the doctrine that strata can be identified by the 

 organic remains which they contain is as sound as when it was 

 first enunciated by William Smith ; but the problems of strati- 

 graphical palaeontology, as they now present themselves to us, 

 are infinitely more complicated than they could possibly have 

 seemed to him. In every fixuna and flora which we are called 

 upon to study, we have to resolve a function of three variables, 

 these beinT environment, space, and time. Only after the most 

 careful investigation, in the first place, of the complicated effects 

 produced by the varied conditions which we group together 

 under the term environment — temperature, food, absence of 

 enemies, and the innumerable influences which, as we now 

 know, determine the existence and affect the multiplication of 

 living beings ; and by the thorough study, in the second place, 

 of the laws of geographical distribution of plants and animals' 

 can we hope to eliminate the effects due to environment and 

 position, and arrire at the conclusion of what must be ascribed 

 to time. 



The task will be long, the work to be done arduous, and the 

 efforts to be made prodigious and sustained ; but the result is 

 one which is not hopeless and unattainable, or, indeed, even 

 doubtful. But let us by all means remember that the real work 

 is really only just commenced, and that we are very far indeed 

 from our goal. 



One of our greatest sources of danger to the progress of 

 geological knowledge at the present day is the impatience which 

 is so frequently shown at the rate of that progress, an impatience 

 which leads to attempts to cut the tangled skeins of research by I 



hasty and ill-considered speculation. Geologists, no less than 

 biologists, need to recollect and keep ever before their minds the 

 important fact that the geological record, although it is one of 

 enormous value, is exceedingly imperfect, and that this imper- 

 fection is quite as conspicuous in respect to physical as it is to 

 palaiontological data. How sadly is this important truth lost 

 sight of by those who, on the strength of a few isolated facts 

 and fragmentary observations, are prepared to construct maps of 

 large portions of the earth's surface at far distant periods of its 

 history. Such maps are to the geologist what "genealogical 

 trees " are to the biologist — " will-o'-the wisps " leading us aside 

 from the safe paths of scientific induction. 



It is, I suspect, from the obvious failure of attempts of this 

 kind — attempts which had better never have been made — that 

 such frequent suga;estions of revolt against the principles of 

 uniformitarianism take their origin. For myself, instead of dis- 

 appointment, I feel a constant surprise that these doctrines have 

 enabled us to explain so much, when our knowledge of the 

 causes still at work around us is still so imperfect ; and I am 

 continually impressed by the fact that each new discovery con- 

 cerning the present order of Nature removes old difficulties in 

 the explanation of the past. In saying that I adhere to the 

 doctrines of uniformitarianism, I, of course, mean the uniformi- 

 tarianism which Lyell himself taught, and not the absurd 

 travesty of that doctrine sometimes ascribed to him. 



The well-grounded conviction which results from observing 

 the triumph of a great principle, when applied in an overwhelm- 

 ing number of cases, and which refuses to abandon that 

 principle at the first appearance of difficulty, is surely not out of 

 place in a student of Nature. It was this scientific "faith" 

 which led Scrope to believe, in spite of difficulties arising from 

 the imperfect knowledge in his day of physics, chemistry, and 

 mineralogy, that massive and schistose crystalline rocks have 

 been formed from ordinary lavas and sediments, when subjected 

 to enormous pressures and complicated earth-movements ; which 

 induced Lyell to seek for and find the key to physical changes 

 during past times in the operations going on everywhere around 

 us ; and which finally conducted Darwin, by the application of 

 the same principle, in the case of living beings, to the doctrine 

 of organic evolution. 



But, alas ! this " faith " seems often sadly wanting among us 

 to-day. At a time when the mineralogical constitution of rocks 

 and of the changes which they undergo is becoming daily more 

 clearly revealed, when innumerable researches are throwing fresh 

 light on the great physical processes taking place everywhere in 

 the world around us, and when each department of biological 

 science is contributing new "facts and arguments for Darwin," 

 such scientific pusillanimity on the part of geologists seems, to 

 say the least of it, singularly inopportune. 



Doubtless there are difficulties still unresolved ; bnt does not 

 every advance in our knowledge see the removal of some of 

 them ? True the task of interpreting the fragmentary record of 

 the rocks is one the end of which seems very far off"; but is not 

 every step we take clearly an approximation towards that end ? 



If any arguments were needed in favour of the continued and 

 close co-operation of geologists and biologists, it would be found 

 in the circumstance that the most important step in the progress 

 of scientific thought which has been accomplished in modern 

 times has been the direct result of a combination of geological 

 and biological researches. 



That remarkable biography, for which we are so greatly in- 

 debted to Mr. Francis Darwin, is not simply the record of a 

 life, simple, blameless, and noble beyond that of ordinary men, 

 the story of the workings of an intellect, truth-loving, patient, 

 and powerful, above that of all his contemporaries ; it is the 

 history of a most wonderful revolution in human thought — one 

 which will perhaps be regarded in future times as the most 

 striking event of the nineteenth century. 



The grand secret of Darwin's success in grappling with the 

 gre.it problem of "the origin of species " is fmmd in the fact 

 that he was at the snme time a geologist and a biologist. The 

 concentration of the later years of his life upon zoological and 

 botanical researches has led many to forget the position occupied 

 by Darvvin among geo'ogists. Not only are his geological 

 writings of the highest value for the wealth of accurate observa- 

 tions which they contain, and the important generalizations 

 which they put forward ; but in his more purely biological works 

 the value of his geological training and experience are constantly 

 exemplified. 



