214 EXPERIMENTAL EVOLUTION LECT. 



concerns anatomical characters : "In the females of 

 the Chadschibai Lake the penicilli or fascicles of knob- 

 bed setae (Tast-Borsteri) are but little developed, being 

 nearly fifty times shorter than the antennae themselves, 

 while in the females of fresh water the same sensitive 

 penicilli are moderately long and only six times 

 shorter than the entire antennae. In the males the 

 sensitive rods are also shorter than in those males 

 inhabiting fresh water. The small hooks situated 

 near the sensitive rods on the tips of the male 

 antennae of fresh-water forms are strongly curved 

 with pointed tips, while in the males of Chadschibai 

 Lake those hooks are shorter, less curved, and with 

 blunt tips. Of the two pointed pale sensory threads 

 situated on geniculated protuberances of the first 

 posterior third section of the male antennae, the 

 posterior one is a little shorter than the anterior 

 thread, the latter coming out a little more in front. 

 These threads are, in the males of Dqphnia rectirostris 

 of the Chadschibai Lake, not in a straight but in a 

 screw-like line. The distance between the threads is 

 considerable, which character in the fresh-water males 

 is much less prominent." I should scarcely have 

 ventured to quote the foregoing lines, and to enter into 

 such very minute details, if it was not a fact familiar 

 to all that many zoologists are to be found in all 

 countries who spend their life in establishing new 



