ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 305 



There were nine outside and three inside. When food was withheld 

 more individuals passed into the interior. It is suggested that Kerona 

 is rather a symbion than a parasite. 



Heritable Variations in Fission Rate of Stylonichia pustulata.* 

 A. R. Middleton has ruade an experimental study of the effects of 

 selection on the fission rate within a single " clone " (pure race). His 

 specific problem was to determine whether it was possible to obtain, by 

 selection from the progeny of a single individual, two types that differ 

 characteristically from each other under identical conditions, and that 

 retain these differences from generation to generation. He found that 

 in S. pustulata it was possible, by the opposite selection through more 

 than 150 generations of small individual variations occurring among 

 the progeny of a single individual, to produce two sets differing 

 hereditarily in rate of fission. During selection there was a gradual 

 increase in the average heritable difference between the two sets, 

 showing that the effect of selection was cumulative. This result was 

 subjected to the most rigid tests possible, by balanced selection 

 throughout long periods, by mass culture without selection, and by 

 reversed selection. In every case the results were corroborated. The 

 hereditary differences induced continued through periods of balanced 

 selection lasting longer than the periods of direct selection by which 

 they were induced, and did not disappear save under the effects of 

 reversed selection. These results were first reached with the progeny of 

 a single individual multiplying asexually. They were confirmed by : — 



(1) Beginning anew with a single individual from this race, and 



(2) experiment with a wild individual quite unrelated to the first two 

 series. In all cases the results were the same. In the third series 

 conjugation occurred, and it was found that the hereditary differences 

 persisted through and after conjugation. The selection of small 

 variations such as appear within the " pure strain " is thus an effective 

 evolutionary procedure. 



Life-history of Didinium nasutum.f — (1. W. Calkins has investi- 

 gated the life-history of Didinium nasutum, with special reference to 

 the problem of the significance of encystment in ciliated Protozoa. 

 The species was chosen for its large size, its easily controlled feeding- 

 habits, and its readiness to encyst. Individuals were placed in fresh 

 water along with Paramoecium, on which the species feeds. If one 

 Didinium and nine Paramoccium were placed together, at the end of 

 twenty-four hours there were usually eight Didiniwn and the Paramoecium 

 had disappeared. The extraordinary process of the seizure and inges- 

 tion of the Parammcium is described and figured. The actively rotating 

 carnivorous Protozoon swims vigorously through the water, making 

 vicious jabs downwards or sideways till it hits something soft enough 

 for its proboscis to penetrate. The whole process is apparently fortuitous, 

 but a Paramecium once hit rarely gets away. It is partially or wholly 

 paralyzed and is speedily swallowed, the walls of the Didinium stretching 



* Jouru. Exper. Zool., xix. (1915) pp. 451-504 (17 figs.). 



t Journ. Exper. Zool., xix. (1915) pp. 225-40 (1 pi. and 12 figs.). 



