202 K. S. LASHLEY 



V. DISCUSSION OF RESULTS 



The experiments reported show that populations of Hydra 

 viridis consist of races which have different hereditary consti- 

 tutions. The diversity between two such strains persisted for 

 so long as they were kept under observation (143 days) and so 

 long as the different strains were kept under similar and favor- 

 able conditions they showed no tendency to approach each other 

 in character. The distinguishing characters of the different 

 races studied were not, however, fixed in the sense of remaining 

 constant through fluctuations in the environment but under- 

 went changes corresponding to changes in the environment. 

 In general, the diverse clones responded to such changes in the 

 same way and to the same degree, although in the face of very 

 unfavorable conditions the larger strain was most affected and 

 under conditions of almost complete starvation the strains 

 became much more similar. 



Little evidence has been obtained as to the cause and funda- 

 mental nature of the difference between the clones. The diverse 

 characters noted, number of tentacles, size, color, and age at 

 which asexual reproduction is begun, may all be modified by 

 changing the environmental conditions and the changes thus 

 produced in the first three characters mentioned are correlated 

 in the same way that they are in the diverse races (large size, 

 many tentacles, and fight color occur together) so that it is pos- 

 sible that the diversities in these three characters are due to a 

 difference in some single set of physiological processes. Failure 

 to obtain sexual reproduction in H. viridis has made it impossible 

 to determine the relation of the diverse races to gametic processes 

 and to the supposed life cycle of Hydra but there is certainly no 

 relation between the diversity of the clones and the phenomena 

 of 'depression' which have been thought to mark periods in the 

 life cycle. 



The existence of diverse races of Hydra is in accord with the 

 results of Jennings, Woltereck, Shull, Whitney and Agar gained 

 from the study of clones of other invertebrates and, indeed, the 

 volume of evidence from zoological and botanical literature 



