C. DOBBLL 155 



the race D (upper line). The largest individual has a length of 256 /x, 

 the smallest 80 ix. But the races all breed true to their mean dimensions. 

 " Breeding from the extreme specimens — ;the largest and smallest — of 

 a .single race, we get several hundred individuals from each. Both 

 produce progent/ of the same mean size. Each pi'oduces a whole series 

 of varying individuals, just like the original racial series ; the series 

 produced by the largest individual is exactly like that produced by 

 the smallest, or by any other. Selection within the pure race is of no 

 effect on the size" (Jennings, 1909). All the races appear to be 

 " singularly resistant, remaining quite constant in most respects, so 

 far as has been determined" (Jennings, 1911). The importance of 

 these conclusions for every worker engaged in the study of genetics 

 in the Ciliata is obvious. There can be little doubt that Jennings's 

 conclusions will be found to hold good for ciliates other than Para- 

 mecium. He himself (cf. Jennings and Hai-gitt, 1910) has shown that 

 there are many indications pointing to this conclusion : and he has also 

 made it clear that non-recognition of the existence of pure lines in 

 general populations of ciliates is a source of error which can lead — and 

 probably has led — to many wrong conclusions concerning variation in 

 these organisms. 



66. Many interesting observations have been made upon the pro- 

 duction and asexual transmission of variations, and the more important 

 of these may now be considered. We may begin with certain normal 

 "modifications" which are due to differences of temperature. R. Hertwig 

 found that the value of the karyoplasmic ratio (§ 30) in a given species 

 of ciliate varies according to the temperature at which the organisms 

 are cultivated'. As a general rule, the size of the meganucleus is 

 relatively larger in animals kept at a low temperature, smaller in those 

 at a high temperature. Further, this difference is both relative and 

 absolute, for the organisms themselves are larger at a low temperature 

 than at a high temperature. 



67. Confirmation of Hertwig's conclusions (§66) has been given by 

 Popotf (1908, 1909) and Rautmann (1909), for Frontonia, Stylonychia, 

 and Paramecium. An illustration may be taken from the case of 

 Stylonychia (Popoff, 1908), summarized in the following table ^: 



■ The value of the karyoplasmic ratio also appears to be different in different races of 

 Paramecium. See note by Popoff and Rautmann (Popoff, 1909, p. 180). 



^ L, B and W stand for length, breadth and width. The length of the nucleus is the 

 combined length of its two components, the width the mean of the two widths. The units 

 of measurement are not microns but di%'isions of the ocular micrometer used by Popoff. 

 I have given the measurements to the first decimal place only. 



