VARIATION AND HEREDITY 179 



operation, and found that operated mouth or teeth were restored to 

 normal form in 3 or 4 generations and that three factors appeared to 

 determine the character and number of teeth: namely, the size of the 

 mouth, the number and arrangement of teeth in the parent, and 

 "something in the constitution of the clone (its genotype) which 

 tends toward the production of a mouth of a certain size, with teeth 

 of a certain form, arrangement, and number." 



Numerous strains have been recognized in all intensively studied 

 parasitic Protozoa such as Entamoeba, Trypanosoma, Plasmodium, 

 etc. For example, Dobell and Jepps (1918) noticed five races in 

 Entamoeba histolytica on the basis of differences in the size of cysts. 

 Spector (1936) distinguished two races in the trophozoite of this 

 amoeba. The large strain was found to be pathogenic to kittens, 

 but the small strain was not. Meleney and Frye (1933, 1935) and 

 Frye and Meleney (1939) also hold that there is a small race in 

 Entamoeba histolytica which has a weak capacity for invading the 

 intestinal wall and not pathogenic to man. Sapero, Hakansson and 

 Louttit (1942) similarly notice two races which can be distinguished 

 by the diameters of cysts, the division line being 10m and 9/i in living 

 and balsam-mounted specimens respectively. The race with large 

 cysts gives rise to trophozoites which are more actively motile, ingest 

 erythrocytes, culture easily and is pathogenic to man and kitten, 

 while the race with small cysts develops into less actively motile 

 amoebae which do not ingest erythrocytes, difficult to culture, and 

 is not pathogenic to hosts, thus not being histozoic. 



Recent investigations by Boyd and his co-workers show beyond 

 doubt that the species of Plasmodium are composed of many strains 

 which vary in diverse physiological characters. In an extended study 

 on Try-pavosoma lewisi, Taliaferro (1921-1926) found that this flagel- 

 late multiplies only during the first ten days in the blood of a rat after 

 inoculation, after which the organisms do not reproduce. In the adult 

 trypanosomes, the variability for total length in a population is about 

 3 per cent. Inoculation of the same pure line into different rats some- 

 times brings about small but significant differences in the mean size 

 and passage through a rat-flea generally results in a significant vari- 

 ability of the pure line. It is considered that some differences in 

 dimensions among strains are apparently due to environment (host), 

 but others cannot be considered as due to this cause, since they per- 

 sist when several strains showing such differences are inoculated 

 into the same host. Hoare (1943) reviewed recently the "biological" 

 races of certain parasitic Protozoa. 



Jollos (1921) subjected Paramecium caudatum to various environ- 



