118 CARNEGIE INSTITUTION OF WASHINGTON. 



MacDowell, from his more extensive numerical series, concludes that, 

 at the beginning, selection as parents of individuals with the greatest 

 number of bristles isolates a many-bristle-producing germ-plasm. 

 But the maximum potentiality in this respect is reached in about 8 

 generations, after which the somatic selection does not result in the 

 isolation of a germ-plasm mth the potency of still more bristles. The 

 effect of somatic selection on the quality of the germ-plasm of the popu- 

 lation is thus nil after the germ-plasm has been rendered homozygous 

 in consequence of the selection of the first 8 generations. 



Experiments with pigeons. — In our pigeon pens Dr. Riddle has dis- 

 covered two fully pigmented but ''weak" individuals, male and female 

 (in whose immediate ancestry or collaterals no whites have appeared) 

 that have become progenitors of lines in which for four generations par- 

 tially white pigeons have appeared. Inbreeding and "crowded repro- 

 duction" have been practiced. In a second hne, pigeons of partially 

 white plumage occurred in one branch of the ancestry. Inbreeding, 

 "crowded reproduction," and selection have been practiced in this 

 series also and in the later generations birds with increased white in the 

 plumage have arisen. In both of these lines, however, the darkest 

 forms have also been selected for breeding, and from these forms also 

 partially white birds have been produced; and greater amounts and 

 proportions of white were obtained, as in the cases cited above, from 

 the later eggs of the the season; i. e., from those obtained under 

 "crowded reproduction." 



THE SIGNIFICANCE AND CONTROL OF SEX. 

 SEX IN PIGEONS. 



Dr. Riddle has continued his studies on sex in pigeons. The results 

 of these studies he has recently stated in the following terms : 



"The studies that have thus far been made on sex, and on the experimental 

 control of sex, in pigeons go very far, we believe, toward an adequate demon- 

 stration that germs prospectively of one sex have been forced to produce an 

 adult of the opposite sex; that germs normally female-producing have, under 

 experiment, been made to develop into males; and that germs which were 

 prospectively male-producing have been made to form female adults. That 

 neither selective fertilization, differential maturation, or a selective elimina- 

 tion of ova in the ovary can account for the observed results. Further, and 

 perhaps of more importance, these studies throw much new light on the nature 

 of the difference between the germs of the two sexes. This difference seems 

 to rest on modifiable metabolic levels of the germs; males arise from germs 

 at the higher levels, females from the lower; and such basic differences are 

 quantitative rather than qualitative in kind." 



Support for Dr. Riddle's views as to the significance of sex are 

 believed by him to be given by certain recent discoveries. Thus he 

 finds, first, that in pigeons a high proportion of the males that result 

 from crosses between genera bear testes whose size and shape relations 

 are reversed from the condition normal for males and approach the 



