RESULTS OF CONTINUED SELECTION IN HYDRA 



K. S. LASHLEY 



From the Zoological Laboratory of the Johns Hopkins University . 



In a recent paper^ I reported an attempt to obtain a modified 

 clone of Hydra viridis by the continued selection of variates dif- 

 fering in the number of tentacles. In that experiment a clone 

 was bred from a single wild polyp and two groups of its descend- 

 ants, each composed of 25 lines, were selected for variations in 

 tentacle number in opposite directions from the mean of the 

 clone. Selection was continued for several generations, then the 

 number of tentacles of all buds produced by the last selected gen- 

 eration of the 50 lines was recorded and the averages of the two 

 groups were compared. A slight difference, in the direction of 

 selection, was found in the averages of the earlier buds of the 

 two groups but this difference did not persist in the buds pro- 

 duced later by the same parents; regression was complete in a 

 single generation. 



The chief criticism of such a negative result in a selection ex- 

 periment is based upon the supposition that in a single character 

 of a species some continuous variations are, others are not in- 

 herited. If this is true, any selected individual may be either a 

 germinal variate, in which case it will contribute to the progres- 

 sive change in the racial character; a somatic variate, in which 

 case its selection will not alter the racial type ; or a variate in 

 which both somatic and germinal variations are combined but 

 active in opposite directions, thus leading to an effect of selection 

 the reverse of that expected from the somatic character. It is 

 further assumed that somatic variation follows more frequently 

 than it contradicts germinal variation, giving natural selection 

 an opportunity to produce evolution when applied to a large 



^ Inheritance in the Asexual Reproduction of Hydra. Jour. Exp. Zool., vol. 

 19, pp. 157-210, 1915. 



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