692 



SCIENCE 



[N. S. Vol. XXVII. No. ( 



fertilization the descendants of a hybrid con- 

 stitute, after a few generations, a population 

 consisting of 2, 4, 8 and so on, fixed types, each 

 constituting half, a fourth, an eighth and so 

 on, of the population, depending on the number 

 of character pairs considered, the number of the 

 fixed types resulting being equal to two with 

 an exponent equal to the number of character 

 pairs. Along with these fixed types is found 

 a decreasing proportion of hybrids between the 

 fixed types. With promiscuous cross fertiliza- 

 tion and with no selection to type the propor- 

 tion of the various types occurring in the 

 second generation of the hybrid remains the 

 same in later generations. 



By that method of selection in which ex- 

 tracted recessives are discarded it was shown 

 that both with cross fertilization and close 

 fertilization the population gradually ap- 

 proaches the type selected. In close-fertilized 

 plants the approach is rapid, making in ten 

 generations a nearly pure population. With 

 promiscuous cross fertilization this method of 

 selection tends to produce a pure population, 

 but at a much slower rate than with close 

 fertilization. Eor instance, with two char- 

 acter pairs, when cross fertilization occurs, in 

 the tenth generation the type selected becomes 

 approximately 65 per cent, of the population, 

 while under similar conditions, with close 

 fertilization, the type selected becomes ap- 

 proximately 99 per cent. 



With animals another type of selection is 

 practicable, viz., that in which one of the 

 sexes is tested for gametic purity and only 

 homozygotes of the type desired used. When 

 this method is used the selected type increases 

 even more rapidly than with close fertilization 

 and elimination of extracted recessives. 



In plants still a diiferent process is prac- 

 ticable, viz., that of testing each second gen- 

 ration individual by saving its seed and plant- 

 ing them separately. When the next crop 

 matures the character of the progeny of the 

 various individuals enables the breeder to 

 select those which are homozygote and those 

 thus establish at once a pure breed. 



The speaker also referred to the so-called 

 elementary species secured by Nilsson, of 



Sweden, and Shull, of this country, explain- 

 ing them as the descendant of a completely 

 homozygote individual. It is claimed that in 

 these completely homozygous forms all heredi- 

 tary variation is completely eliminated and 

 that only such variation occurs as is due to 

 environment. 



Dr. C. O. Townsend asked whether the 

 speaker considered variation caused by en- 

 vironment to be hereditary. In answer it was 

 stated that that question had not been suffi- 

 ciently investigated to permit of a categorical 

 answer. When plants are moved to an en- 

 tirely new environment, important variations 

 frequently occur. Some of these variations 

 are hereditary, at least under the new environ- 

 ment. Whether thes^ would be permanent 

 should the descendants of the plant be re- 

 turned to the original environment has not 

 been sufficiently investigated. 



Mr. M. 0. Marsh read the second paper, 

 describing " A New Method of Marking Sal- 

 mon Fry." Distinctive marks on young sal- 

 mon, provided they do not interfere with the 

 life and movements of the fish and are recog-- 

 nizable in the adult, are very useful in ac- 

 quiring information concerning the age and 

 migrations of salmon and the efficiency of 

 artificial propagation. The Bureau of Fish- 

 eries has obtained definite returns from sal- 

 mon fry marked in Alaska by the amputation 

 of certain fins with scissors. These marks 

 were placed upon fry about three months old. 

 The operation of marking is rather slow and 

 tedious. The possibility of rapidly marking 

 sahnon fry in the sac stage or at the time of 

 planting, by cauterizing the fins with a 

 thermo-cautery needle was shown. Either the 

 adipose, dorsal or anal may be removed in this 

 way and probably other fins. The shock of 

 the operation causes a moderate loss which 

 ceases after two or three days and the remain- 

 ing fry recover. Evidence thus far gives 

 reasonable assurance that regeneration will not 

 interfere with the recognition of the marked 

 fish which return as adults. Combination 

 marks on the same fish may to some extent 

 be made and in this way the number of dis- 

 tinctive marks may be increased. A given 



