(26) 



multiply freely. 



During this time more than 175 offspring were produced 

 by each group. The average spine numbers of these two sets 

 of progeny differ by nearly a whole spine, with a probable 

 error so small as to be negligible. This experiment, I 

 think, proves conclusively that the effect of selection 

 which has been demonstrated in experiment 3 is due to the 

 inheritance of minute variations in spine number within 

 the clone. 



In this mass selection experiment, the appearance 

 of a mutation has little effect on the ultimate result. 

 Something comparable to a mutation did, in fact, appear 

 in the low series of experiment 4. During the first 

 period of multiplication, individual Idl, with one spine, 

 began producing progeny all of which had no spines. Pro- 

 geny with no spines were comparatively rare among the other 

 low lines. All these progeny of Idl also produced progeny 

 with no spines, predominantly, but with a few one-spined 

 progeny. The early appearance of this mutation gave it 

 greater prominence than it would have enjoyed later on, 

 since Idl and three of its offspring were included among 

 the progenitors of low lines. In all, 39 individuals can 

 be traced back to Idl as their original ancestor. Of these, 

 33 had no spines and 6 had one spine. But the effect on 

 the final result of the experiment is inconsiderable. If 

 all of the offspring of tlie mutation are left out of the 

 table, the number of individuals produced by the low lines 

 between August 26 and September 3 is reduced to 168, with 



