636 



NATURE 



[July 14, 1921 



Adjustments are provided for bringing the second 

 principal point of the lens under test on to the axis 

 ^bout which it is rotated by the bar, and also for 

 bringing the centre of curvature of the mirror exactly 

 on the axis of the roller above-mentioned and on 

 the optical axis of the lens. The distance from the 

 ■centre of the roller in the axial position to the axis 

 ■of rotation of the lens is measured by a vernier. 

 When all adjustments have been made, this vernier 

 gives the focal length of the lens to an accuracy of 

 about 0001 in. 



The apparatus measures the degree to which the 

 wave-front, impressed by the lens on light from a 

 ■distant point source, differs from a spherical wave- 

 front. The indications are given in aberrations of 

 wave-front to a scale of wave-lengths, the aberration 

 shown being in every case twice that present in the 

 once transmitted beam which normally forms the 

 image of a distant point. The form in which the 

 indications are presented is that of a series of inter- 



ference fringes which are lines of equal aberration of 

 waye-front. These interferometer pictures can be 

 translated into terms of geometric optics by an ob- 

 server who has had a little practice with the instru- 

 ment. The various types of aberration and their 

 chromatic variations produce characteristic interfer- 

 ence patterns, and thus they can be readily differen- 

 tiated and measured in terms of wave-length. By 

 means of the pair of deflectors on a measurement of 

 the distortion can also be obtained. 



With a suitable source of light and a suitable 

 camera the interference patterns can be photographed, 

 and a complete photographic record can be obtained 

 of the performance of any camera lens. Fig. 3 is 

 a photographic reproduction of the interferograms of 

 a well-known lens of high repute for the green mer- 

 curv radiation (546/x/x) for the axial beam and for 

 obliquities of 5°, 10° and 15°. It will be seen that 

 even the best photographic lenses— of which this is 

 a fair example— are very far indeed from perfection. 



Mutations and Evolution. 



I 



N the series of articles by Dr. Ruggles Gates 

 appearing under the above title in a iVew 

 Phytologist Reprint (No. 12), published by Messrs. 

 Wheldon and Wesley, Ltd., we have the most 

 recent attempt to present a reasoned and com- 

 prehensive statement of the problem of evolution. 

 As the author tells us, his aim has been to I 

 show that though germinal (by which apparently 

 we may understand chromosomal) changes are of 

 importance in the evolutionary process, they can- 

 not be considered as all-sufficing ; that only from 

 the Neo-Lamarckian point of view is it possible 

 to explain a large class of organic phenomena. From 

 this point he sets out to show how the Darwinian 

 ■doctrine and Mendelian conceptions in combination 

 may furnish us with a solution. To this end, how- 

 -ever, it scarcely seems necessary to maintain, as the 

 author is at pains to reiterate, that in the application 

 •of Mendelian principles we are merely putting into 

 use a refinement of the theory of natural selection. 

 Nor does any point app>ear to be gained by this insist- 

 •enoe on accord, since, by the author's own showing, 

 the underlying difference between Darwinism and 

 Mendelism — the difference, namely, between the idea 

 of continuity and discontinuity — is profound enough 

 to have divided biologists into two opposite camps. 

 •One feels that what is common ground might more 

 easily be made apparent if an attempt were made to 

 define more strictly, or else to abandon, terms which 

 are used to cover an ever-increasing complex of ideas. 

 It will be obvious, for example, that a fresh analysis 

 of evolutionary processes should be couched in terms 

 which clearly differentiate the causes ( = true factors) to 

 which variation is presumably due from the 

 mechanism by which variations, once having ap- 

 peared, are perpetuated, and from conditions which 

 permit or limit the occurrence of variation. That the 

 author evidently has in mind the necessity for pre- 

 cision in this connection appears from the fact that 

 he is careful to point out that isolation due to geo- 

 graphical barriers must be regarded as a condition, 

 and not as a factor, yet he fails to draw this distinc- 

 tion when dealing with natural selection. 



The important point which Dr. Gates seeks to 

 establish is that a new character may arise in two 

 different ways : (i) as the result of what we have still 

 to term spontaneous nuclear ( = karyogenetic) muta- 

 tions ; (2) from a so-called organismal change, i.e. a 

 •change due either to environmental effects on the cyto- 



NO. 2698, VOL. 107] 



plasm or to the morphological principle known as 

 orthogenesis. In the first case the mutation is per- 

 petuated through the whole cell-lineage, and the asso- 

 ciated character is inherited as a unit. In the second 

 a localised region or a particular stage in the life- 

 cycle only is usually affected. Perpetuation of an 

 organismal modification connotes the inheritance of 

 acquired characters. 



Mutations. — The more striking observations of 

 Morgan and other American workers on Drosophila 

 and of de Vries, the author, and others on CEnothera, 

 which indicate a direct relation between chromosomal 

 behaviour and somatic appearance, are set forth. 

 Definite zygote characters are shown to be constantly 

 associated with definite irregularities in the meiotic 

 division, as, e.g., the lata habit in CEnothera with the 

 presence of an extra chromosome. The author brings 

 forward evidence of independent sporadic appearances 

 of this form, and a parallel mutation has been obtained 

 in cultures of other Oenothera species. In every case 

 the number of chromosomes was found to be 15 

 instead of the typical complement 14. The occasional 

 occurrence of an 8-6 instead of a 7-7 separation of 

 the chromosomes in another mutant form supplied 

 the clue to the mode of origin of these 15-chromosome 

 forms. In another instance a particular strain of 

 Drosophila, indistinguishable in general from the 

 normal but showing an aberrant type of inheritance, 

 led Bridges to infer the duplication of a sex-chromo- 

 some — a prediction which later investigation proved 

 to be correct. These forms with an extra chromosome 

 are found seldom, if ever, to breed true. Their im- 

 portance, according to the author, lies in the support 

 which they give to the conception of the origin of a 

 zygotic character from a nuclear mutation rather 

 than in their significance in evolution. It is held to 

 be otherwise, however, when the whole chromosomal 

 equipment is duplicated (tetraploidy) and associated 

 with a characteristic giant habit as in Primula and 

 CEnothera. 



The separate class of Mendelian mutations is re- 

 garded as due also to a nuclear change (in this case 

 possibly chemical) which is presumed, however, to 

 affect only a particular locus or element in the chromo- 

 some. It is clear, however, from Bridges 's observation 

 cited above, and from Heribert-Nilsson's work on Salix 

 (which the author does not discuss), that, on one 

 hand, duplication of chromosomes need not be accom- 

 panied by any gross change in the organism, and, on 



