November 15, 1917] 



NATURE 



215 



fact that large changes occur as well as small ones; 

 this seems perhaps due to the fact that we are wit- 

 nessing- the disintegration of highly developed appa- 

 ratus in place of its building up. 



In all this, except the last point, the work on Droso- 

 phila is in agreement with my own observation of 

 gradual variation in Difflugia, with Castle's similar 

 results on the rat, and with the conclusions of palaeon- 

 tologists as to the gradual development of the char- 

 acteristics of organisms in past ages. 



But there is one point in the pala;ontological con- 

 clusions which is not in agreement with the experi- 

 mental and observational results on existing organ- 

 isms; this I wish to notice briefly. Osborn sets forth 

 that in following given" stocks from earlier to later 

 ages, characters arise from minutest beginnings, and 

 pass by continuous gradations to the highly developed 

 condition ; these developing characters do not show 

 random variations in all directions, but follow a 

 definite course, which might seem to have been in 

 some way predetermined. And this is emphasised by 

 the fact that the same sorts of characters (horns, for 

 ; example) may arise independently, at different ages, 

 in diverse branches of the same stock, and each follow 

 in later ages the same definite course of development. 

 Evolution is characterised by Orthogenesis, as this 

 phenomenon has sometimes been called. 



Now it appears to me that we do not observe this 

 in the present-day experimental work ; by selection we 

 can move in more than one direction. There is no 

 indication, so far as I can see, that the variations 

 push in one determinate direction only. Examining 

 the palaeontological summaries further as regards 

 this, we find that diverse courses are followed by given 

 characters, in diverse branches of a given group. 



A second point which Osborn sets forth is deserving 

 of particular attention. He states, in agreement with 

 Waagen, that in any given geologic stratum, we do 

 tind, in addition to characteristics that are in the line 

 of determinate descent, other variations from this line, 

 which are of the sort that constitute what we call at 

 the present time varieties; things that are like the 

 diverse races of DifHugia in my own work. But, say 

 Osborn and Waagen, there is' a great difference in 

 jjrinciple between these and the others, for those which 

 are in the determinate line of progress persist into the 

 next geologic stratum, while the mere varieties do not. 

 The persistent changes were called by Waagen muta- 

 tions (in a sense somewhat diverse from that in which 

 the word is used by de Vries). 



Osborn expresses the opinion that these "varieties" 

 may be merely non-heritable modifications. i" But in 

 our present geologic period we find just such diverging 

 forms, in great number, and we find that their pecu- 

 liarities are heritable. There is, then, no reason for 

 supposing that these variations were not heritable in 

 earlier geological periods ; there must have been many 

 races heritably diverse, just as there are now; and 

 these are what Waagen called varieties. 



Now, since this is so, the onlv difference between 

 Waagen 's mutations and his varieties is that the 

 former persisted and the latter did not. But this tells 

 us nothmg whatever about why .the latter did not. It 

 is perfectly possible, so far as these facts go, that it 

 was a matter of selection by external conditions ; manv 

 liverse stocks were present, on an equal footing'; 

 some were destroyed, others were not. The conditions 

 described by the palaeontologists support strongly the 

 theory of evolution by gradual change, but I cannot 

 see that they tend to establish the view that varia- 

 tions show a tendency to follow a definite course, as 

 if predetermined. The palaeontologists appear rather 

 to report precisely the conditions which we are bound 

 to find if evolution occurs through the guidance of 



10 Osborn, 1915, p. 225. (See Bibliography.) 



NO. 2507, VOL. 100] 



natural selection operating on a great numl»er of 

 diverse variations, the typical Darwinian scheme. 



There is one other point, made by Bateson (1914), in 

 his presidential address before the British Association, 

 and further developed by Davenport (1916) in a recent 

 paper : the proposition, namely, that since practically 

 all observed variations are cases of loss and disintegra- 

 tion, we are driven to suppose that evolution has 

 occurred by loss and disintegration. Davenport com- 

 bines this idea with the theory that these disintegrat- 

 ing variations follow a definite course, predetermined 

 in large measure by the constitution of the disintegrat- 

 ing material. 



There are two points worth consideration in dealing 

 with this theory. The first is one of fact; although; 

 it is true that many of the so-called mutations appear 

 to be cases of loss and disintegration, yet there is nO' 

 indication that this is the case in such effects of selec- 

 tion as have been described by Castle and myself; 

 variations are not limited to any particular direction. 

 Secondly, it appears to me that this conclusion — that 

 because the variations we see are cases of loss and 

 disintegration, therefore evolution must have occurred, 

 by loss and disintegration, involves an error in logic, 

 which makes it unworthy of serious consideration. 



To summarise, then, what I have obtained from 

 experimental work combined with a survey of the work 

 of others, the impression left is as follows : — 



(i) Experimental and observational study reveals, 

 that organisms are composed of great numbers of 

 diverse stocks differing heritably by minute degrees. 



(2) Sufficientlv thorough study shows that minute 

 heritable variations — so minute as to represent prac- 

 tically continuous gradations — occur in many organ- 

 isms, some reproducing from a single parent others toy 

 biparental reproduction." 



(3) The same thing is reported from palaeontological 

 studies. 



(4) On careful examination we find even that the 

 same thing is revealed by such mutationist work as 

 that on Drosophila ; single characters exist in so many 

 grades due to minute alterations in the hereditary con- 

 stitution as to form a practically continuous series. 



(5) It is not established that heritable changes must 

 be sudden large steps ; while these may occur, minute 

 heritable changes are more frequent. 



(6) It is not established that heritable variations 

 follow a definite course as if predetermined ; they occur 

 in many directions. 



(7) It is not established that all heritable changes are 

 by disintegration ; although many such do occurj they 

 cannot be considered steps in progressive evolution 

 from the visibly less complex to the visibly more com- 

 plex. 



Evolution according to the typical Darwinian 

 scheme, through the occurrence of many small varia- 

 tions and their guidance by natural selection, is per- 

 fectly consistent with what experimental and palaeon- 

 tological studies show us ; to me it appears more con- 

 sistent with the data than does any other theory. 

 Bihlibgra^hy. 



W. Bateson (1914), Address of the President of the 

 British Association for the Advancement of Science, 

 Science, vol. xl., pp. 319-33. C. B. Bridges (1916), 

 Non-disjunction as proof of the chromosome theory of 

 heredity. Genetics, vol. i., pp. 1-52, 107-63. W. E. 

 Castle (1915), Mr. Muller on the constancy of Men- 

 delian characters, Amer. Nat., vol. xlix., ,pp 37-42. 

 W. E. Castle (19150), Some experiments in mass selec- 

 tion, Amer. Nat., vol. xlix., pp. 713-726. W. E. 

 Castle (1916), Can selection cause genetic change? 

 Amer. Nat., vol 1., pp. 248-56. W. E. Castle (1916a), 

 Further studies of piebald rats and selection, with ob- 

 servations oft gametic coupling, Carnegie Institution, 

 Washington, Pub. 241, part iii., pp. 161-92. W. E. 



