May 20, 1922] 



NA TURE 



641 



circulate. The arthropods have a heart (except in 

 some modified groups), a pericardium, and a large 

 system of haemocoelic spaces ; the Crustacea have 

 definite arterial vessels. The arthropods, again, 

 breathe by gills borne on the limbs or by tracheae. 

 But what Mr. Clark means by the respiratory system 

 of the echinoderms is not clear : they aerate their 

 body fluids in so many ways ; such structures as 

 papulae, when developed, are not much like anything 

 in arthropods. As for the crystalline skeleton of the 

 echinoderms, deposited in the spaces of an inter- 

 penetrating mesoderm and resorbed as need arises, 

 it can only be contrasted with the chitinous cuticle 

 of the arthropods, hardened by the deposition of 

 amorphous lime salts, and incapable of modification 

 except through moulting. Lastly, how can the radial 

 segmentation of echinoderms be homologised with the 

 metameric segmentation of the arthropods ? Mr. 

 Clark does not tell us. He does, however, accept the 

 general view that " the echinoderms are derived from 

 bilaterally symmetrical ancestors," and he does later 

 on quote Patten's hypothesis that the original meta- 

 meres of the ancestor were suppressed on one side, 

 and the remainder of the body bent round into a ring, 

 so that the half metameres with their segmental 

 organs became arranged in radiating lines — a hypothesis 

 which places a very strained interpretation on the 

 facts of embryology, and seems inconsistent with 

 Mr. Clark's own views as to the relations of larva and 

 adult. 



No one would wish to assert that a study of the 

 development and mode of life of the cirripedes can 

 have no bearing on the origin of the echinoderms. 

 There is much that is suggestive in the comparisons 

 drawn by Patten and by Clark. But if those writers 

 mean to conclude that the echinoderms were derived 

 from the cirripedes by way of the crinoids, then as- 

 suredly they have been misled by adaptive resemblances. 

 If they mean only that these resemblances imply a 

 likeness of ancestral material no less than a likeness 

 of external conditions, then we must ask them to 

 indicate the connection between the ancestor of the 

 Crustacea (nauplius-, Apus-, or trilobite-like, as they 

 choose) and the bilaterally symmetrical ancestor 

 (Dipleurula or what not) of the Echinoderma. It is 

 quite possible that there was a primitive group of 

 ccelomate animals from which the early echinoderms 

 and the chaetopod ancestors of the arthropods both 

 arose ; but to imagine that the arthropod type, once 

 evolved on the " appendicular plan," as Mr. Clark 

 calls it, retraced its steps towards anything that could 

 have become an echinoderm, is contrary to all ascer- 

 tained principles of evolution ; and the alternative 

 dream, that an arthropod, once recognisable as such, 

 NO. 2742, VOL. 109] 



could progressively change into an echinoderm, is a 

 baseless and unsubstantiated vision. 



Mr. Clark will not, it is to be hoped, think we dis- 

 miss him in summary fashion. Many will say we ought 

 to have done so. But there is a reason for treating 

 the matter seriously. Any one has a perfect right 

 to discuss the origin of the echinoderms and to main- 

 tain what views he pleases. Mr. Clark, as a distin- 

 guished authority on one class of echinoderms, certainly 

 may claim a hearing. But whoever discusses morpho- 

 logical problems should have regard to the recognised 

 principles and methods of morphology. He should 

 have a sufficiently wide knowledge of comparative 

 anatomy to be able to estimate the relative values of 

 the. facts that he adduces. There is at the present 

 time a real danger that this discipline may be for- 

 gotten in the rush after alluring discovery in genetics, 

 biochemistry, and other novel branches of biology. 

 Among many examples of loose thinking in morphology, 

 this of Mr. Clark occupies a bad eminence, exaggerated 

 by the place of its publication and by the high merit 

 of its author in his own field. F. A. Bather. 



Physics for Students. 



(i) An Outline of Physics. By L. Southerns. Pp. 

 XV + 202. (London : Methuen and Co., Ltd., 1920.) 

 6^. 6d. 



(2) General Physics and its Application to Industry and 

 Everyday Life. By Prof. E. S. Ferry. Pp. xvi + 

 732. (New York : J. Wiley and Sons, Inc. ; London : 

 Chapman and Hall, Ltd., 192 1.) 245. net. 



(3) Laboratory Projects in Physics : A Manual of 

 Practical Experiments for Beginners. By F. F. 

 Good. Pp. xiii + 267. (New, York: The Mac- 

 millan Co. ; London : Macmillan and Co., Ltd., 

 1920.) 95. net. 



(4) An Introduction to Physics for Technical Students. 

 By P. J. Haler and A. H. Stuart. Pp. 240. (London : 

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(5) Experimental Science, i. Physics. By S. E. 

 Brown. Section 5, L/^R Pp. vii -1-273-424. (Cam- 

 bridge : At the University Press, 1920.) 6s. net. 



(6) Elements of Natural Science. By W. Bernard 

 Smith. Parti. Pp. viii + 207. (London: Edward 

 Arnold, 1921.) 55. net. 



(i) ]\ /r R- SOUTHERNS has faced the difficulty, 

 IVX which many university lecturers have 

 experienced, of providing a course in physics for college 

 students in their first year. Not only are there great 

 differences in the preliminary knowledge with which 

 the students are equipped, but also different groups — 

 say, engineers and medicals — look at the subject from 

 very different angles. The first year should be a year 



