164 



NATURE 



[September 30, 1920 



An occasional experiment along some other line has 

 not been enough to obscure the general trend ; indeed, 

 the speedy scrapping of such failures only emphasises 

 the idea of a determined plan. But closer considera- 

 tion shows that the course of the development was 

 guided simply by the laws of mechanics and economics 

 and by the history of discovery in other branches of 

 science. That alone was the nature of the deter- 

 mination, and predetermination there was none. 

 From these instances we see that selection can, indeed 

 must, produce just that evolution along definite lines 

 which is the supposed feature of orthogenesis. 



The arguments for orthogenesis are reduced to 

 two : first, the diflficulty of accounting for the 

 incipient stages of new structures before they achieve 

 selective value ; and, secondly, the supposed cases of 

 non-adaptive, or even — as one may term it — counter- 

 adaptive, growth. 



The earliest discernible stage of an entirely new 

 character in an adaptive direction is called by H. F. 

 Osborn a " rectigradation " (1907), and the term im- 

 plies that the character will proceed to develop in a 

 definite direction. Osborn gives as instances the first 

 folding of the enamel in the teeth of the ancestral 

 horses and the first slight elevation on the skull of 

 the older Titanotheres, foreshadowing the large nose- 

 horns of those strange Tertiary mammals. He con- 

 trasts rectigradations with the changes in shape and 

 proportion of some pre-existing structure, and calls the 

 latter "allometrons." Further, he claims that some 

 predetermining law or similarity; of potential governs 

 the appearance of rectigradations, because they arise 

 independently on the same part of the skull in different 

 lineages at different periods of geological time. 



Osborn maintains, then, that rectigradations are a 

 result of the principle of determination, but this does 

 not seem necessary. , In the first place, the precise 

 distinction between an allometron and a rectigradation 

 fades away on closer scrutiny. When the rudiment of 

 a cusp or a horn changes its form, the change is an 

 allometron ; the first swelling is a rectigradation. 

 But both of these are changes in the form of a pre- 

 existing structure; there is no fundamental difference 

 between a bone with an equable curve and one with 

 a slight irregularity of surface. Why may not the 

 original modification be due to the same cause as the 

 succeeding ones? The development of a horn in 

 mammalia is probably a response to some rubbing or 

 butting action which produces changes first in the 

 hair and epidermis. One requires stronger evidence 

 than has yet been adduced to suppose that in this 

 case form precedes function. .\s Jaekel has insisted, 

 skeletal formation follows the changes in the softer 

 tissues as they respond to strains and stresses. In 

 the evolution of the Echinoid skeleton any new- struc- 

 tures that appear, such as auricles for the attachment 

 of jaw-muscles and notches for the reception of 

 external gills, have at their inception all the character 

 of rectigradations, but it can scarcely be doubted that 

 thev followed the growth of their correlated soft 

 parts, and that these latter were already subject to 

 natural selection. But we may go further; in verte- 

 brates, as in Echinoderms, the bony substance is inter- 

 penetrated with living matter, which renders it 

 directly responsive to every mechanical force, and 

 modifies it as required by deposition or resorption, so 

 that the skeleton tends continually to a correlation of 

 all its parts and an adaptation to outer needs. 



The fact that similar structures are developed in 

 the same positions in different stocks at different 

 periods of time is paralleled in probably all classes of 

 animals; Ammonites, Brachiopods, Polyozoa, Crinoids, 

 and Sea-urchins present familiar instances. But do we 

 want to make anv mystery of it? The words "pre- 



NO. 2657, VOL. 106] 



disposition," "predetermining law," "similarity of | 

 potential," "inhibited potentiality," and " [>criodicity " 1 

 all tend to obscure the simple statement that like 

 causes acting on like material produce like effects. 

 When ottier causes operate the result is different. 

 Certainly such facts afford no evidence of predeter- 

 mination in the sense that the development must take 

 place willy-nilly. Quite the contrary ; they suggest 

 that it takes place only under the influence of the 

 necessary causes. 



The resemblance of the cuttle-fish eye to that of 

 a vertebrate has been explained by the assumption 

 that both creatures are de.scende<l, longo intervallo no 

 doubt, from a common stock, and that the flesh or 

 the germ of that stock had the internal impulse to 

 produce this kind of eye some day when conditions 

 should be favourable. It is not explained why many 

 other eyed animals, which must also have descended 

 from this remote stock, have developed eves of a 

 different kind. Nevertheless, I commend this hypo- 

 thesis of Prof. Bergson to the advocates of pre- 

 disposition. To my mind, it only shows that a philo. 

 sopher may achieve distinction by a theory of evolu- 

 tion without a secure knowledge of biologA-. 



When the .same stock follows two quite different 

 paths to the same goal it is impossible to speak of 

 a predetermined course. [.An instance of this was 

 given.] 



(To be continued.) 



The Constitution of Cellulose- 



1 N an illuminating lecture delivered before the 

 '■ PVench Chemical Society on .May 21, Prof. A. 

 Pictet, of Geneva, described the results obtained bv 

 his pupils and himself on distilling cellulose at a low 

 pressure, and showed how these can be interpreted so 

 as to throw much new light on the constitution of 

 this complicated substance. 



When cotton cellulose is heated gradually in a dis- 

 tilling apparatus under a pressure of 10-15 mm. de- 

 com|X5sition begins at 210° and an oil distils over 

 equal in weight to 45 per cent, of the original cellu- 

 lose, which soon solidifies, and consists of Isevo- 

 glucosan. This is considered to be an anhydride 

 derived from ^-glucose, and to have the constitution 



CH(OH) . CH . OH 

 I I 



CH — O— CH 



I ! 



Previous work has shown that cellulose furnishes 

 ort acetolysis a disaccharide, cellobiose, which probably 

 contains an a-glucose and a /S-glucose group. .Mso, 

 with hydrobromic acid, cellulose gives bromomethyl- 

 furfurol. The origin of the latter, a hydrofuran 

 nucleus containing two side-chains, the author terms 

 the chitose grouping. Prof. Pictet therefore regards 

 cellulose as containing two /3-glucose groups, one 

 chitose grouping, and probably an a-glucose group, 

 represented thus : 



3 |Ch 



3 !«?■ 



By acetolysis the a-glucose group and a /3-glucose 

 group together form cellobiose (50 per cent.), and in 

 the decomposition with hydrobromic acid the chitose 

 grouping furnishes bromomethylfurfurol (25 per cent.), 

 the other three groups being converted into the black 

 mass which is always formed in the reaction. Finally, 

 on dry distillation under reduced pressure the /3-gluc6se 

 groupings split ciff to give Iftvoglucosan (50 per cent.). 



