April 23, 1920] 



SCIENCE 



417 



that an increased growth accompanied the het«ro- 

 lygous condition. All the essential facts were ao- 

 eounted for. A decade later the great extension of 

 knowledge in the field of heredity has made pos- 

 sible a still closer linking of the facts of imbreed- 

 ing and outbreeding with Mendelism. The hy- 

 pothesis of the complementary a<ition of dominant 

 factors is the logical outgrowth of former views 

 and makes the increased growth of hybrids some- 

 what more understandaible. The fact of a stimu- 

 lation accompanying heterozygosity is supplemented 

 by a reason why such an effect is obtained. The 

 former view of a physiological stimulation and the 

 more recent conception of the combined action of 

 dominant factors are not then two unrelated hy- 

 potheses io be held up for the choosing of the one 

 from the other. The outstanding feature of the 

 latter view is that there is no longer any question 

 as to whether or not inbreeding as a process in 

 itself is injurious. Homozygosity, when obtained 

 with the combination of all the most favorable 

 characters, is the most effective condition for the 

 purpose of growth and reproduction. 



A chapter on the value of inbreeding and 

 outbreeding in plant and animal improvement 

 gives a very sane and well-balanced discussion 

 of the practical application in agriculture of 

 the principles set forth in the earlier portion 

 of the work. So far. as thoroughly scientific 

 exposition may hojw to do so the bogey of the 

 necessary and inevitable harmfulness of in- 

 breeding is laid to rest. It is pointed out 

 that, so far as may be judged from the past, 

 inbreeding has been the greatest single in- 

 striunent in the breeder's hands for securing 

 imiformity and the concentration of desirable 

 qualities. It has the further advantage of 

 bringing clearly to light undesirable qualities 

 which may then be easily eliminated by selec- 

 tion or otherwise. 



The last two chapters of the book are of a 

 more si)eculative character, but surely no one 

 will deny to those who have made such solid 

 experimental deposits in the bank of knowl- 

 edge the right to speculate a bit. The first 

 of these chapters deals with effects upon the 

 individual and the second with effects upon 

 the race. Both chapters may fairly be re- 

 garded as among the sanest and most cogent 

 arguments for the integral incorpioration of 

 eugenic ideas and ideals into the conduct of 



the social and political affairs of life which 

 have yet been put forth. The known facts 

 are examined critically, though briefly, and 

 there is a refreshing absence of blind and 

 blatant propaganda. To take a single simple 

 example it is shown with great clearness that 

 the ridding of a racial germ-plasm of defective 

 characters is very far from being the simple 

 process that enthusiastic devotees of steriliza- 

 tion legislation would have us believe. To 

 prevent the multiplication of individuals 

 visibly bearing the defects is, in theory at 

 least, not particularly difficult. But to do 

 this alone will not even approximately solve 

 the problem. The residual and vastly more 

 difficult question concerns the somatically 

 normal transmitters of defective qualities. 



Altogether this is a notable book, in which 

 American science may well take pride. It 

 should form a part of the required reading 

 of every student of biology, because nowhere 

 else is there brought together in such clear 

 and well-digested form the results of a mass 

 of experimental work which has successfully 

 lighted a dark corner of biological science. 

 Raymond Pearl 



SPECIAL ARTICLES 



CORRESPONDENCE BETWEEN CHROMOSOME 



NUMBER AND LINKAGE GROUPS IN 



DROSOPHILA VIRILIS 



A STUDY of twenty-seven mutant characters^ 

 in Drosophila virilis Sturtevant, reveals the 

 presence of at least five groups of linked genes 

 in this species — in contrast to the four groups 

 in Drosophila melanogaster (ampelophila) . 

 This diiTerence in number of linkage groups 

 agrees in a significant manner with the differ- 

 ence in number of chromosomes in the two 

 species. D. melanogaster, as is well known, 

 has four pairs of chromosomes — three large 

 and one very small — and correspiondingly has 

 three large groups and one small group of 

 linked genes. B. virilis, on the other hand, has 

 six pairs of chromosomes — five large and one 



1 Descriptions of some of these have appeared in 

 two earlier papers: Metz;, C. W., Genetics, 1: 591- 

 607, November, 1916, and Metz, C. W., ihid., 3: 

 107-134, March, 1918. 



