520 



SCIENCE 



[N. S. Vol. XXVin. No. 720 



animals. In other places I have attempted to 

 show the need of definite reco^ition of the 

 fact that the transmission of characters is 

 quite distinct from expression.* 



The spread of a character through a group 

 by transmission does not appear to have any 

 relation to the frequency -with which the char- 

 acter comes into visible expression. In their 

 ability to spread through species recessive 

 characters have a distinct advantage over dom- 

 inant characters. In the presence of an ad- 

 verse selection a recessive or latent character 

 could continue to spread, even in spite of the 

 elimination of all the individuals in which the 

 character came into expression, whereas a 

 dominant character would be destroyed as soon 

 as its representatives were exterminated. 



It is also known that the potency, or power 

 of a character to come into expression, is sub- 

 ject to pronounced changes, even among dif- 

 ferent individuals of the same stock. Thus 

 one of Professor Davenport's tailless fowls 

 produced only tailed chicks, though the 

 Mendelian reckoning called for large per- 

 centages of tailless birds. And yet the tailless 

 character reappeared in Mendelian proportions 

 in the progeny of a son of the same bird.° 



Thus the biological probabilities regarding 

 brachydactyly are altogether different from the 

 mathematical calculations based on the Men- 



^ " Transmission Inheritance distinct from Ex- 

 pression Inheritance," Science, N. S., XXV., 911. 

 " Mendelism and Other Methods of Descent," 

 Proc. Wash. Academy of Sciences, IX., 189. 

 "Heredity Related to Memory and Instinct," 

 Monist, XVIII., 263. 



- " Altogether, out of 200 offspring of this tail- 

 less cock, where I expected 90 per cent, tailless 

 birds, I got not one. On the other hand, using 

 some of the same hens with another cock (the 

 son of No. 117), from 50 offspring, where I ex- 

 pected 25 tailless, I got 24 tailless. In No. 117, 

 although tailless, the tailed tendency strongly 

 dominates over taillessness, so that not in the 

 first nor in the second hybrid generation does 

 taillessness appear, and of the Mendelian segre- 

 gation in the second hybrid generation there is 

 no trace! On the other hand, another cock re- 

 veals typical Mendelian phenomena." See Daven- 

 port, C. B., 1907. " Heredity and Slendel's Law," 

 Proc. Washington Academy of Sciences, IX., 184. 



delian assumption that parental characters 

 are transmitted by only half of the germ-cells. 

 The biological indication is that brachydactyly 

 is transmitted to all the descendants of a 

 brachydactylous ancestor, and is likely to re- 

 gain expression, or even to become prepotent, 

 in any generation, near or remote. 



O. F. Cook 

 Washington, 

 July 16, 1908 



SCIENTIFIC BOOKS 

 A Text-Booh of the Principles of Animal 

 Histology. By Ulric Dahlgeen, Assistant 

 Professor of Biology in Princeton Uni- 

 versity, and William A. Kepnee, Adjunct 

 Professor of Biology in the University of 

 Virginia. Pp. xiii + 515. Price, $3.75. 

 New York, Macmillan Company. 1908. 

 This book is so unlike the usual text-books 

 of human and mam m alian histology that it 

 will seem like an entirely new subject to most 

 readers. It comes as a welcome relief from 

 the multitude of text-books which differ from 

 one another only in the order and arrange- 

 ment of the subjects treated. For many years 

 the comparative method has been recognized 

 as the " saving salt," as Michael Foster ex- 

 pressed it, of anatomy and embryology, but 

 strange to say, few works have attempted to 

 deal with histology from the comparative 

 point of view, and this subject has been ade- 

 quately treated only in the case of man and 

 of a few mammals. If we except the early 

 pioneer work of Leydig and the incompleted 

 work of Fol, the only works which deal spe- 

 cifically and adequately with the subject of 

 comparative histology are the large manual of 

 Camillo Schneider and this volume by Dahl- 

 gren and Kepner, and the present work is, I 

 believe, the first attempt which has been made 

 in English to put histology upon a compara- 

 tive basis. 



The purpose of the authors is clearly stated 

 in the preface to be 



To produce a work that covers the general field 

 of histology, and is not restricted in the main to 

 human and mammalian forms. It is intended to 

 be a work that teaches general principles and 



