October 7, 1920] 



NATURE 



19: 



ihis lij4ht, even the most extravagant spines lose their 

 mystery and appear as consequences of natural 

 selection. 



The fact that many extreme deselopments are fol- 

 lowed by the extinction of the race is due to the 

 ditticulty that any specialised organism or machine 

 finds in ad>ipting itself to new conditions. .\ highly 

 specialised creature is one adapted to quite peculiar 

 circumstances ; very slight external change may put 

 it out of harmony, especially if the change be sudden. 

 It is not necessary to imagine any decline of vital 

 force or exhaustion of potentiality. 



What, then, is the meaning of "momentum" in 

 evolution? Simply this: that change, whatever its 

 cause, must be a change of something that already 

 exists. The changes in evolving lineages are, as a 

 rule, orderly and continuous, hnvironment changes 

 slowly and the response of the organism always lags 

 behind it, taking small heed of ephemeral variations. 

 Suppose a change from shallow to deep water— either 

 by sinking of the sea-floor or by migration of the 

 organism. Creatures already capable of becoming 

 acclimatised will Ik- the majority of survivors, and 

 among them those which change most rapidly will 

 scion dominate. Place your successive forms in order, 

 ;iiul you will get the appearance of momentum; but 

 tli<- rciility is inertia yielding with more or less rapidity 

 to an outer force. 



• « ♦ 



But in all these apparent instances we should do 

 \\<11 to realise that we arc still incompletely informed 

 al)out the daily life of these creatures and of their 

 ancestors in all stages of growth, and we may 

 remember that structures once adaptive often persist 

 after the need has p-issed or has been replaced by 

 one acting in a dilTerent direction. 



The Study of Adaptive Form. 



I his leads us on to consider the influence of the 

 •li! of life on the shape of the creature, or, briefly, 

 Junction on form ; and, conversely, the indications 

 • form can give as to habits and habitat. For 

 ly a long year the relatively simple mechanics of 

 vertebrate skeleton have been studied by paUeonto- 

 >ts and anatomists generally, and have been 

 ight into discussions on the effect of use. These 

 lies, however, have usually considered the struc- 

 of an animal as an isolated machine. \Vc ihave 

 •lalise that an organism should be studied in rela- 

 te the whole of its environment, and here form 

 'S in as distinct from structure. .Similar adaptive 

 ^ins are found in organisms of diverse structure, 

 produce those similarities which we know as 

 nvergencc." To take but one simple instance from 

 relations of organisms to gravity. .\ stalked 

 liinoderm naturallv grows upright, like a flower, 

 radiate symmetry. But in th« late Ordovician 

 in Silurian rocks are many in which the body 

 ta curiously flattened leaf-like shape, in which (he 

 I faces are distinct but the two sides alike, and in 

 ch this effect is often enhanced by paired out- 

 Vths corresponding in shape if not in structure, 

 insion of this kind implies a position parallel to 

 learth's surface, i.e. at right angles to gravity. 

 I«af-like form and tlx- balancers are adaptations 

 his unusual position. Recognition of this enables 

 interpret the peculiar features of each genus, to 

 rate the adaptive form from the modifn-d struc. 

 and to perceive that many genera outwardly 

 W are really of quite different origin. 

 Stil we umlerstand the principles governing 

 and other adaptations — irrespective of the svs- 

 |tlc position nf the creatures in which they appear 

 cannot make adequate reconstructions of our 

 NO. 2658, VOL. 106] 



fossils, we cannot draw correct inferences as to their 

 mode of life, and we cannot distinguish the adaptive 

 trom the fundamental characters. No doubt many of 

 us have long recognised the truth in a general way, 

 and have attempted to describe our material — whether 

 in stone or in alcohol — as living creatures; and not 

 as isolated specimens, but as integral portions of a 

 mobile world. It is, however, chiefly to Louis Dollo 

 that we ow-e the suggestion and the example of ap- 

 proaching animals primarily from the side of the 

 environment, and of studying adaptations as such. 

 The .inalysis of adaptations in those cases where the 

 stimulus can be recognised and correlated with its 

 reaction (as in progression through different media 

 or over different surfaces) affords sure ground for 

 inferences concerning similar forms the life-conditions 

 of which we are ignorant. But from such analyses 

 there have been drawn wider conclusions pointing to 

 further extension of the study. It was soon seen that 

 adaptations did not come to perfection all at once, 

 but that harmonisation was gradual, and that some 

 species had progressed further than others. But it by 

 no means follows that these represent chains of 

 descent. The adaptations of all the organs miist be 

 considered and one seriation checked by another. 



In applying these principles we are greatly helped 

 by Dollo's thesis of the Irreversibility of Evolution. 

 This is a simple statement of the facts as hitherto 

 observed, and may be expressed thus : 



(i) In the course of race-history an organism never 

 returns exactly to its former state, even if placed in 

 conditions of existence identical with those through 

 which it has previously passed. Thus, if through 

 adaptation to a new mode of life (as from walking to 

 climbing) a race loses organs which were highly 

 useful to it in the former state, then, if it ever reverts 

 to that former mo<le of life (as from climbing to 

 walking) those' organs never return, but other organs 

 are modified to take their place. 



(2) But (continues the law), by virtue of the in- 

 destructibility of the past, the organism always pre- 

 serves some trace of the intermediate stages. Thus, 

 when a race reverts to its former state there remain 

 the traces of those modifications which its organs 

 underwent while it was pursuing another mode of 

 existence. 



The first statement imposes a veto on anv specula- 

 tions as to descent that involve the reappearance of. a 

 vanished structure. The Second statement furnishes a 

 guide to the mo<ie of life of the immediate ancestors, 

 and is applicable to living as well as to fossil forms. 

 It is from such persistent adaptive characters that 

 some have inferred, the arboreal nature of our own 

 ancestors, or even of the ancestors of all mammals. 



The Study of Habitat. 



The natural history of marine invertcbrata is of 

 particular interest to the geologist, but its studv 

 presents peculiar diflficulties. The marine zoologist has 

 long recognised that his early efforts with trawl and 

 dredge threw little light on the depth in the sea 

 frequented by his captures. The surface floaters, the 

 swimmers of the middle and lower depths, and the 

 crawlers on the bottom were confused in a single 

 haul, and he has therefore devised means for explor- 

 ing each region separately. TIh- geologist, however, 

 finds all these faunas mixed in a single deposit. He 

 may even find with them the winged cre.iiures of the 

 air, as in the insect beds of Gurnet Bay, t>r the remains 

 of estuarinc and land animals. 



The I'pper Ordovician starfish bed of Girvan con- 

 tains not only the crawling and wriggling creatures 

 from which it takes its name, but also stalked cchino- 

 derms adapted to most varied modes of life, swim- 



