548 H. S. Jennings and George T. Hargitt. 



that the variations in form could not be used as differentiating 

 marks of species or varieties, but were called forth by variations 

 in the surrounding medium and changed not only from place to 

 place, but also from time to time." 



Of course another view is possible; that the different forms 

 really belong to different lines or strains, one strain replacing 

 another by hatching out of the eggs as the environment favor- 

 able for development of this strain appears. These two views 

 are discussed by Wesenberg-Lund; he points out that evidence 

 for deciding between them is in most cases quite lacking. Which 

 view is correct can only be determined by breeding experiments 

 with carefully isolated lines. The work of Wesenberg-Lund 

 furnishes a vast mine of suggestions for most interesting experi- 

 mental work. Breeding experiments with pure lines should be 

 carried on with such polymorphic forms as Anuraea cochlearis 

 and Brachionus bakeri among the rotifers; with the Daphnias, 

 Cyclops, Ostracoda, among the Crustacea, etc. Different lines 

 should be subjected to the same environment, to see if they 

 become identical; different parts of the same line to different 

 environments, to see how far they take on different forms. The 

 work thus far done with pure lines suggests the probability that 

 the differing forms will be found much more constant than had 

 been supposed, many permanently differentiated races existing, 

 though differing environments will cause variations of form in 

 each race, but in a rather limited degree. This appears to be the 

 view to which Wesenberg-Lund himself has gradually come, 

 though he feels the need of experimental evidence. One or two 

 investigations along this line on such lower animals should be 

 mentioned. Hanel ('07) reported that many different lines of 

 Hydra grisea exist, characterized by having different average 

 numbers of tentacles, though the number within each line varies 

 with environmental conditions. Hase ('09) however has recently 

 denied that the number of tentacles is hereditary at all, thus 

 doing away with the existence of these diverse lines. 



Woltereck ('08) examined two local races of Daphnia longi- 

 spina and found that the differences were permanent and heredi- 

 tary, even when the races were cultivated for two years under the 



