150 Racial Studies m Fishes 



being a fluctuation of only one-tenth of a degree (± 0*1). The two 

 parents had respectively 7 and 5 rays in the dorsal fin. The experi- 

 ment took place partly at 25°, partly at 18°. The three first broods 

 were produced at 25°, following which the parents were maintained at 

 18°, from the day the third brood was born until the birth of the fourth 

 brood. After this the temperature was raised again to 25°, at which 

 degree the development of the last broods of young took place. The 

 result of the experiment is given in Table VI. 



TABLE YI. 



Number of dorsal rays in offspring of the same pair of parents at 

 different temperatures. 



No. of Specimens 



25° 25° 25° 18° 25° 25° 



^^:Pi Brood 1 Brood 2 Brood 3 Brood 4 Brood 5, 6, 7 Brood 1, 2, 



3, 5, 6, 7 



rays in 



offspring Born 15/5 1918 Born 10/6 1918 Born 6/7 1918 Born 21/9 1918 Born 25/10, 21/11, 

 V ^ ' 24/12 1918 



7 8 13 29 6 51 101 



6 1 2 3 13, 4 10 



5 _ _ _ 1 — — 



n 9 15 32 20 55 . Ill 



a 6-889 6-867 6*906 6-250 6*927 6910 



a ±0-333 ±0*352 ±0-296 ±0-550 ±0*262 ±0-283 



P.E.A. ±0-075 ±0*061 ±0-035 ±0-083 ±0024 ±0018 



Fl. ±0*375 ±0*306 ±0*177 ±0-415 ±0*129 ±0«092 



Thus we see, that whilst the broods developed at 25° had an average 

 of 6*91 rays, the average number of rays fell to 6*25 at 18°, The 

 difference between the averages was thus 0*660 and the probable error 

 of this difference ± O'OSo. 



As all experiments in this connection have given a similar result, it 

 may be taken as proved that the number of rays in the dorsal fin of the 

 offspring is affected to a considerable degree by the temperature to which 

 the mother is subjected whilst in a state of pregnancy. Remarkable 

 besides is the great difference in the duration of pregnancy at the 

 different temperatures; which at 25° lasted ca. 1 month, at 18° more 

 than 3 months. 



III. Importance of Internal Factors. 



The object of the experiments about to be discussed was to investi- 

 gate whether hereditary differences in the number of dorsal fin rays 

 could be proved to exist. The principle of the experiments was to 

 maintain different pairs of parents in the same environment, and see 

 whether the offspring were different as regards the number of rays. 



