NOTICES OF NEW BOOKS. 357 



much is due to variability in the environmental conditions of development. 

 3. Characters that are indistinguishable— e.g. whiteness in poultry may 

 be produced Ijy different genes. 4. Each character is the product of 

 many genes, but each of these may change witliout the others changing. 

 Both in segregation and in crossing over each pair is inseparable from 

 the others. 



Of mutations it may be said that they appear infrequently, that the 

 change is definite from the beginning, that some at least are recurrent, 

 and that the difference between the old character and the new one is 

 small in some cases and greater in others. Their origin remains obscure. 

 As to their supposed " chance " character, it is pointed out that the 

 degree of development of any character increases the probability of 

 further stages in the same direction. Species are to be thought of as 

 groups of genes, and related species have a good many genes in common. 

 Thus similar mutations are likely to occur in different species, and there 

 is experimental evidence of this in Drosophila. J. A. T. 



Traite de la Lumike. Par Christian Huyghens. 1920. 155 x x pp. 

 Price 3 fr. GO. Published by Gauthier-Villars et Cie., Paris. 



Microscopical Preparations. Catalogue of Zoological and Botanical 

 Mateiial, Fresh and Preserved. 1920. Flatters and Garnett, Ltd., 

 309 Oxford Road, Manchester. 



Report of the Enquiry Committee on the Standardization of the 

 Elements of Optical Instruments. 39 pp. Price Is. net. Pub- 

 lished for the Department of Scientific and Industrial Research by 

 His Majesty's Stationery Office. 



National Physical Laboratory Report for the Year 1919. 152 pp. 



Price 5s. net. Published by His Majesty's Stationery Office. 



Common Diatoms. P>y Thomas K. Mellor, F.R.A.S. 1920. 16 pp., 

 7 pis. Price Qs. net. Published by William Wesley and Son, 

 28 Essex Street, Strand, W.C. 



Studies on Acari. No. 1. The Genus Demodex Owen. By Stanley 

 Hirst. 1919. 44 pp., 13 pis. Price lOs. Published by the British 

 Museum (Xatural History), Cromwell Road, S.W.7 



Marine Boring Animals. By W. T. Caiman, D.Sc. 1919. 36 pp. 

 Price Is. Published by the British Museum (Natural History), 

 Cromwell Road, S.W.7 



The British Charophyta. By .James Groves, F.L.S., and George 

 Russell Bullock-Webster, M.A., F.D.S. 1920. 142 pp., 20 pis. 

 Published by the Ray Society. 



A Monograph of the British Orthoptera. By William John Lucas, B.A. 

 1920. 264 pp., 25 pis. Published by the Ray Society. 



