HEREDITY AND VARIATIONS IN PROTOZOA 



581 



suitable but mysterious environmental conditions which were 

 absent in all other series. 



Series 45 Avith a relative vitality of 105.1 per cent was another 

 and a similar case of unusual combinations. Like Series 19 it came 

 from a parental series of low vitality (Series 39, vitality 48.9 per 

 cent) and was exceptional in having a high division-rate over a 

 long period. Its peculiarities were not handed on to its offspring 

 (Series 53) . 



Other extraordinary combinations resulting in abnormally weak 

 series may also be regarded as incompatible combinations. These 

 are characterized by having both low initial and low general vitality, 

 the most extreme cases coming from parents of extremely old age. 

 They are of no experimental \alue for they quickly die out (see 



Fig. 2.37. — JJroleptus mohilis; vitality graph of Series 19. 



Table, p. 560). It is significant that the.se exceptional cases all 

 came from old-age individuals with differentiated protoplasm 

 through long-continued metabolic activity and cell division. It 

 appears to be a period when unusual combinations are possible, 

 as with Series 19, 45, etc. It may have been such combinations 

 which resulted in Prandtl's (1906) instances of Didiniwn nasnhnn 

 in which zygote nuclei develop into macronuclei without differentia- 

 tion of micronuclei, or of amicronucleate races generally (Patten, 

 1921). 



Apart from the macronuclei, and type of vitality, there is no 

 evidence of a permanent change of the genot\'pe in Uroleptiis 

 mohilis and there is no evidence at all of permanent mutations, and 

 the same conclusion applies to so-called mutations in Protozoa 



