September 30, 1920] 



NATURE 



'63 



Whatever may be the explanation, the facts do 

 seem to warrant the statement that evolutionary 

 change can be, and often is, continuous. I propose 

 to speak of it as "transition." 



The Direction of Change. 



Those who attempt to classify species now living 

 frequently find that they may be arranged in a con- 

 tinuous series, in which each species differs from its 

 neighbours by a little less or a little more ; they 

 find that the series corresponds with the geographical 

 distribution of the species; and they find sometimes 

 that the change affects particular genera or families 

 or orders, and not similar assemblages subjected, 

 apparently, to the same conditions. They infer from 

 this that the series represents a genetic relation, that 

 each successive species is the descendant of its pre- 

 ceding neighbour ; and in some cases this inference 

 is warranted by the evidence of recapitulation — a fact 

 which further indicates that the change arises by 

 addition or subtraction at the end of the individual 

 life-cycle. For this appearance of successive differ- 

 ences we may here use the brief and non-committal 

 term "seriation." 



The comparison of the seriation of living species 

 and genera to the seriation of a succession of extinct 

 forms as revealed by fossils was first made by Cope, 

 who in 1866 held the zoological regions of to-day to 

 be related to one another "as the different sub- 

 divisions of a geologic period in time." This com- 

 parison is of great importancx-. Had we the seriations 

 of living forms alone, we might often be in doubt as 

 to the meaning of the phenomenon. In the first 

 place, we might ascribe it purely to climatic and 

 similar environmental influence, and we should be 

 unable to prove genetic filiation between the species. 

 Even if descent were assumed, we should not know 

 which end of the series was ancestral, or even whether 

 the starting-point might not be near the middle. But 

 wfien the palaeontologist can show the same, or even 

 analogous, seriation in a time-succession, he indicates 

 to the neontologist the solution of his problem. 



Restricting ourselves to series in which descent 



niav be considered as proved or highly probable, we 



; find then a definite seriation — not merely transition, 



i. but transition in orderly sequence such as can be 



[ represent«-d by a graphic curve of simple form. If 



! there are gaps in the series as known to us, we can 



•safely pr<-dict tJieir discovery; and we can prolong 



the curve backwards or forwards so as to reveal the 



ii.itiire of ancestors or descendants. 



Orthogenesis : Determinate Variation. 



rh<' regular, straightforward character of such 



■ iiation led Eimer to coin the term "orthogenesis" 



! tlw- phenomenon as a whole. If this term be 



lien as purely descriptive, it s«Tves well enough to 



'i-note certain facts. Hut orthogenesis, in the minds 



■f most people, connotes the idea of necessity, of 



U'rminate variation, and of predetermined course. 



\v, just as you may have succession without cvolu. 



n, s<i you may have seriation without determina- 



ri or predetermination. I^t us be clear as to the 



■ .ming of these terms. Variation is said to be 



!'rminate or "definite " when all the offspring vary 



the same direction. .Ml the changes arc of the 



iiie kind, though they may differ in degree. For 



ii^tance, .-ill may consist in some ad<lition, as a 



thickening of skek-tal structures, an outgrowth of 



•-pines or horns; or all may consist in some loss, as 



ttv smaller size of outer digits, the diminution of 



tubercles, or the disJippearance of feathers. A suc- 



resdlon of such determinate variations for several 



NO. 3657, vol.. 106] 



generations produces seriation ; and when the seria- 

 tion is in a plus direction it is called progressive, when 

 in a minus direction retrogressive. Now, it is clear 

 that if a single individual or generation produces 

 offspring with, say, plus variations differing in degree, 

 then the new generation will display seriation. 

 Instances of this are well known. Vou may draw 

 from them what inferences you please, but you can- 

 not actually prove that there is progression. Breed- 

 ing experiments under natural conditions for a long 

 series of years would be required for such proof. 

 Here again the paleontologist can point to the records 

 of the process throughout centuries or millennia, and 

 can show that there have been undoubted progression 

 and retrogression. I do not mean to assert that the 

 examples of progressive and retrogressive series found 

 among fossils are necessarily due to the seriation of 

 determinate variations, but the instances of deter- 

 rninate variation known among the creatures now 

 living show the palaeontologist a method that mav 

 have helped to produce his series. Once more the 

 observations of neontologist and palaeontologist are 

 mutually complementary. 



Predetermination. 

 So much for determination ; now for predetermina- 

 tion. This is a far more difficult problem, discussed 

 when the fallen angels 



reasoned high 

 Of provideoce, foreknowledge, will, and fate, 

 Fixed fate, free will, foreknowicdge absolute, 

 And found no end in wandering mazes lost, 



and it is likely to be discussed so long as a reasoning 

 mind persists. For all that, it is a problem on which 



hmgs which, being 

 continuously moved by a certain principle inherent in 

 themselves, arrive at a certain end." In other words, 

 a race once started on a certain course will persist in 

 that course, no matter how conditions may change, 

 no matter how hurtful to the individual its own 

 changes may be, progressive or retrogressive, uphill 

 and downhill, straight as a Roman road, it will go 

 on to that appointed end. Nor is it onlv paleonto- 

 logists who think thus. Prof. Duerden has recently 

 written: "The Niigelian idea that evolutionary 

 changes have taken place as a result of some internal 

 vitalistic force, acting altogether independently of 

 external influences, and proceeding along definite 

 linos, irrespective of adaptive considerations, seems to 

 be gaining ground at the present time among 

 biologists." 



The idea is a taking one, but is It really warranted 

 by the facts at our disposal ? We have seen, I repeat, 

 {hat succession does not imply evolution, and (grant- 

 ing evolution) I have claimed that seriation can occur 

 without determinate variation and without predeter- 

 mination. It is easy to see this in the case of 

 inanimate objects subjected to a controlling force. 

 The fossil-collector who passes his material through 

 a series of sieves, picking out first the larger shells, 

 then the sm.'tller, and finally the microscopic 

 Foraminifera, induces a seriation in size by an action 

 which may be compared to the selective action 

 of successive environments. TIktc is, in this ca.se, 

 predetermination imposed by an external mind, but 

 there is no dett-rminate variation. You may sec in 

 the museum at Leicester a srri<'s beginning with the 

 i>ia .\trala of the Roman occupants of Britain, and 

 passing through all stages of the tramway up to the 

 engine<'red modern railroad. The unitv and apparent 

 im-vitabilitv of the serk's conjure up the vi.sion of 

 a world-mtnd consciou.sly working to a forcteen t'nd. 



