VARIATION AND HEREDITY 



177 



tinguished 12 such varieties and showed that any variety could be 

 changed into another by using different culture media. This trans- 

 formation, however, did not occur at the same rate among different 



Fig. 85. Varieties of Ceratiuvi hirundinella from various European 

 waters (Schroder), a, furcoides-type (130-300/x by 30-45ju); b, brachy- 

 ceroides-type (130-145ju by 30-45^t); c, silesiaciun-type (148-280/x by 

 28-34/x); d, carinthiacum-type (120-145/x by 45-60/i); e, gracile-type 

 (140-200M by 60-75m); f, austriacum-type (120-160^ by 45-60)u); g, 

 robustum-type (270-310^ by 45-55/i); h, scotticum-type (160-210/x by 

 50-60/i); i, piburgense-type (180-260^ by 50-60)u). 



races. It was found that the longer a strain has remained under con- 

 ditions producing a given type, the greater the time and the number 

 of generations needed to change it to a new type under a new condi- 

 tion, as is shown in Table 7. 



While in many species, the races or varieties have apparently been 

 brought about into being under the influence of environmental con- 

 ditions, in others the inherited characters persist for a long period, 

 and still in others the biotype may show different inherited char- 



