366 C. W. MITCHELL AND J. H. POWERS 



The histories of J and K, on the other hand, are very different 

 and were a great surprise to us at the time the experiment was 

 conducted. The transition from the saccate to the humped type 

 which was so tardy and transient in H and I, appeared in K 

 promptly in the second generation and in J in the third. More- 

 over, when once produced the humped type became singularly 

 constant throughout almost the entire series. But once in J 

 and three times in K did single generations occur in which a part 

 of the individuals only reverted to the saccate type; yet, though 

 no preference was shown in the selection of humped individuals to 

 continue the series at these times of reversion and although no 

 change of food was made, the humped type was immediately 

 resumed in the next generation. 



This constancy of the higher type was in striking contrast to our 

 entire previous experience in the rearing of pedigree series of this 

 rotifer. While in mass culture the humped type is readily main- 

 tained yet in no other instance of a long continued series of iso- 

 lation cultures had it been possible to avoid frequent reversions to 

 the smaller saccate type. The only instance where this was done 

 for a score or more generations^ was where a special food stimulus 

 was applied. That our success with / and K was not due to food 

 supply employed is proven by the fact, which we have already 

 suggested, that the copious food supply was taken from the same 

 source as was that fed to H and I and was given to all in excess of 

 consumption. In size J and K were of course larger than H and 

 /, being of the humped type; the average number of young pro- 

 duced per individual approximated closely to fourteen in / and to 

 fifteen in K. 



A study of the metabolic rhythm of the four series, which may 

 be made out by noting the number of young present in successive 

 generations, shows the relative independence of the four series 

 in regard to each other, and also in regard to external conditions. 

 Their crescendos and diminuendos are not coincident. Never- 

 theless, a closer approach to parallelism obtains between J and K, 

 on the one hand, and between H and 7, on the other, than obtains 

 between any two of the contrasted series. 



^ See article I, table 10, historj' of Series D. 



