194 



NATURE 



[October 7, 1920 



ming and creeping trilobites, and, indeed, representa- 

 tives of almost all marine levels. 



In the study of such assemblages we have to dis- 

 tinguish between the places of birth, of life, of death, 

 and of burial, since, though these may be all the 

 same, they may also all be different. The echino- 

 derms of the starfish bed further suggest that closer 

 discrimination is needed between the diverse habitats of 

 bottom forms. Some of these were, 1 believe, attached 

 to seaweed ; others grew up on stalks above the 

 bottom ; others clung to shells or stones ; others lay 

 on the top of the sea-floor; others were partly 

 buried beneath its muddy sand ; others may have 

 grovelled beneath it, connected with the overlying 

 water by passages. Here we shall be greatly helped 

 by the investigations of C. G. J. Petersen and his 

 fellow-workers of the Danish Biological Station. They 

 have set an example of intensive study which needs to 

 be followed elsewhere. By bringing up slabs of the 

 actual bottom they have shown that, even in a small 

 area, many diverse habitats, each with its peculiar 

 fauna, may be found, one superimposed on the other. 

 Thanks to Petersen and similar investigators, exact 

 comparison can now take the place of ingenious 

 speculation. And that this research is not merely 

 fascinating in itself, but illuminatory of wider ques- 

 tions, follows from the consideration that analysis of 

 faunas and their modes of life must be a necessary 

 preliminary to the study of migrations and geo- 

 graphical distribution. 



The Temfo of Evolution. 



We have not yet done with the results that may 



flow from an analysis of adaptations. Among the 



many facts which, when considered from the side of 



animal structure alone, lead to transcendental theories 



with Greek names, there is the observation that the 



relative rate of evolution is very different in races 



living at the same time. Since their remains are 



found often side by side, it is assumed that they were 



subject, to the same conditions, and that the differences 



of speed must be due to a difference of internal motive 



force. After what has just been said, you will at 



once detect the fallacy in this assumption. Prof. .\bel 



has recently maintained that the varying tempo of 



evolution depends on the changes in outer conditions. 



He compares the evolution of whales, sirenians, and 



horses during the Tertiary epoch, and correlates it 



with the nature of the food. 



» * * 



Whether such changes of food or of other habits of 

 life are, in a sense, spontaneous, or whether they are 

 forced on the creatures by changes of climate and other 

 conditions, makes no difference to the facts that the 

 changes of form are a reaction to the stimuli of the 

 outer world, and that the rate of evolution depends 

 on those outer changes. 



Whether we have to deal with similar changes of 

 form taking place at different times or in different 

 places, or with diverse changes affecting the same or 

 similar stocks at the same time and place, we can 

 see the possibility that all are adaptations to a 

 changing environment. There is, then, reason for 

 thinking that ignorance alone leads us to assume some 

 inexplicable force urging the races this way or that, 

 to so-called advance or. to apparent degeneration, 

 to life or to death. 



The Rhythm of Life. 



The comparison of the life of a lineage to that of 

 an individual is, up to a point, true and illuminating ; 

 but when a lineage first starts on its independent 

 course (which reallv means that some individuals of a 

 pre-existing stock enter a new field), then I see no 

 reason to predict that it will necessarily pass through 



NO. 2658, VOL. 106] 



periods of youth, maturity, and old age, that it will 

 increase to an acme of numbers, of variety, or of 

 specialisation, and then decline througii a second child-' 

 hood to ultimate extinction. Still less can we say 

 that, iis the individuals of a species have their allotted 

 span of time, long or short, so the species or the 

 lineage has its predestined term. The exceptions to 

 those assertions are indeed recognised by the sup- 

 porters of such views, and they are explained in terms 

 of rejuvenescence, rhythmic cycles, or a grand despair- 

 ing outburst before death. This phraseology is de- 

 lightful as metaphor, and the conceptions have had 

 their value in promoting search for confirmatorv or 

 contradictory evidence. But do they lead to any broad 

 and fructifying principle ? When one analyses them 

 one is perpetually brought up against some transcen- 

 dental assumption, some unknown entelechy that 

 starts and controls the machine, but must for ever 

 evade the methods of our science. 



The facts of recurrence, of rhythm, of rise and 

 fall, of marvellous efflorescences, of gradual decline, 

 or of sudden disappearances, all are incontestable. But 

 if we accept the intimate relation of organism and 

 environment, we shall surmise that on a planet with 

 such a geological history as ours, with its recurrence 

 of sipiilar physical changes, the phenomena of life 

 must reflect the great rhjthmic waves that have up- 

 lifted the mountains and lowered the deeps, no less 

 than every smaller wave and ripple that has from 

 age to age diversified and enlivened the face of our 

 restless mother. 



To correlate the succession of living forms with all 

 these changes is the task of the palaeontologist. To 

 attempt it he will need the aid of everv kind of bio- 

 logist, every kind of geologist. But this attempt is 

 not in its nature impossible, and each advance to the 

 ultimate goal will, in the future as in the past, pro- 

 vide both geologist and biologist with new light on 

 their particular problems. When the correlation shall 

 have been completed, our geological systems and 

 epochs will no longer be defined by gaps in our know- 

 ledge, but will be the true expression of the actual 

 rhvthm of evolution. Lyell's great postulate of the 

 uniform action of Nature is still our guide, but we 

 have ceased to confound uniformity with monotony. 

 We return, though with a difference, to the concep- 

 tions of Cuvier, to those numerous and relatively 

 sudden revolutions of the surface of the globe which 

 have produced the corresponding dynasties in its suc- 

 cession of inhabitants. 



The Future. 



The work of a systematic palaeontologist, especially 

 of one dealing with rare and obscure fossils, often 

 seems remote from the thought and practice of 

 modern science. I have tried to show that it is not 

 really so. But still it may appear to some to have 

 no contact with the urgent problems of the world 

 outside. That also is an error. Whether the views 

 I have criticised or those I have supported are the 

 correct ones is a matter of practical importance. If 

 we are to accept the principle of predetermination or 

 of blind growth-force, we must accept also a check 

 on our efforts to improve breeds, including those of 

 man, by any other means than crossings and elimina- 

 tion of unfit strains. In spite of all that we may do 

 in this wav, there remain those decadent races, 

 whether of ostriches or human beings, which " await 

 alike the inevitable hour." If, on the other hand, we 

 adopt the view that the life-history of races is a 

 response to their environment, then it follows, no 

 doubt, that the past history of living creatures will 

 have been determined bv conditions outside their con- 

 trol, it follows that the idea of human progress as a 



