Experimental Evolution, Variation, and Heredity 109 



In the main body of the volume each of the six important species is described 

 and analysed and by the employment of statistical methods colonial differentia are 

 brought to light which would otherwise escape detection. The peculiar features 

 of the description, variation, and genetic relations are emphasized in every case. 

 In briefest form the general results are as follows: (1) The snails are far from 

 uniform in their distribution; with only one exception each group of islands has 

 its own characteristic species which occur nowhere else. (2) The same correlation 

 between geographical and specific discontinuity is displayed by the species of the 

 different islands of one and the same group for each member possesses distinct 

 species not found in the others, although in a very few instances important ex- 

 ceptions occur which throw much light upon the processes of dispersal and migra- 

 tion as well as upon certain geological relations. (3) As in the Achatinellidas, the 

 species of Partula may vary from valley to valley of an island; a form sometimes 

 extends over a wide range, while another may be restricted to a few valleys or 

 even to one; less frequently differences are displayed by types which inhabit dif- 

 ferent parts of one valley. Statistical results prove the essential difference of 

 races belonging to a species that has been heretofore regarded as invariable while 

 in other cases close relationships have been established for species previously con- 

 sidered as separate. The study of the relations between young and parent snails 

 is of the highest importance in establishing these results. (4) The abundant 

 material, taken in connection with the results of Garrett, gives astonishingly clear 

 evidence of a recent origin of some types where it is possible also to determine 

 their parentage and rate of dispersal when this has occurred, as well as the fixity 

 of the new characteristics. (5) Mutation has been demonstrated in numerous 

 instances, and in many species belonging to several islands, so that it can not be 

 regarded as a unique process. (6) The influence of the "environment" seems to 

 be little or nothing. Isolation proves to be a "condition" and not a "factor" in 

 the differentiation of forms belonging to the genus Partula. (7) A result with 

 a wider scientific bearing is established by the comparison with one another of the 

 snail populations of islands of the same group and of the species of different groups. 

 All the evidence goes to show that the dominant geological process in South 

 Pacific regions has been one of subsidence, which has progressively isolated various 

 mountain ranges previously connected so that they have become separate island- 

 masses, which in their turn have been subsequently converted into the disconnected 

 islands of the several groups. 



No. 237. MORGAN, T. H., and C. B. BRIDGES. Sex-linked Inheritance in Dro- 

 sophila. Octavo, 87 pages, 2 plates, 8 figs. Published 1916. Price $1.50. 

 An account of 36 sex-linked mutations in Drosophila ampelophila, of which 15 

 are described here for the first time. The numerical data (obtained in the experi- 

 ments upon the linkage relations of the factors for these mutations) furnish the 

 material by means of which the location of the factors in the sex chromosome 

 is determined. A compilation of all the previously published data on the inter- 

 linkage of the sex-linked factors was made and on the basis of all the available 

 data a new map of the location of the factors in the X chromosome is constructed. 

 An introduction points out how the Mendelian ratios and the peculiar distributions 

 seen in linkage are explicable by means of the chromosome mechanism and states 

 the argument for the chromosome theory of heredity, especially with relation to 

 the inheritance of sex-linked characters and the determination of sex; some of 

 the latest evidence for this theory is given in the discussion of non-disjunction of 

 the sex chromosomes, and some of the general methods of procedure in working 

 with sex-linked characters, such as balanced viability, are explained. Other gen- 

 eral subjects discussed are the determination of multiple allelomorphism ; the 

 detection of sex-linked lethals; the formation of polymorphic races; sterility; 

 and the influence of the environment. 



