No. 549] A CASE OF POLYMORPHISM 



527 



belonged to the other. I will record here also two in- 

 stances in which I have found the cannibalistic trophi 

 in the humped rotifer. The two specimens were almost 

 the last humped individuals found in a culture on the 

 verge of extinction, through cannibalism; they were prob- 

 ably the progeny of campanulates. Both were large 

 specimens of the humped type, one showing a rather heavy 

 corona and rather small humps, and being, therefore, in 

 some sense, of a transitional character. It bore campan- 

 ulate trophi 257/* long — rather undersized — in which, 

 however, the large lamellate teeth of this type were even 

 nn usually developed. The other Wore typical eampanu- 

 late trophi 270/* long, These animals contained large 

 unborn humped rotifers of normal type, with normal 

 trophi of 154/* and 170/*, respectively. In these two in- 

 stances, therefore, the transition from the campanulate 

 to the smaller type occurred one generation sooner in 

 general body form than it did with the trophi, thus em- 

 phasizing the partial separateness and non-correlation 

 between the variations in these differently formed struc- 

 tures. 



Next as to duration of transition periods and the num- 

 ber of transitional individuals. If conditions are favor- 

 able the periods are very brief and the number of transi- 

 tional types so few that they are readily overlooked un- 

 less careful search is made at just the right time. The 

 entire population of a teeming Asplauclnm pond readily 

 changes from the saccate to the humped type in one week. 

 As before said, the saccates give birth directly to forms 

 with well-developed humps, and these humped young may 

 be at birth as large or even larger than the parent type. 

 One more generation of growth and reproduction may 

 then give large-sized, fully typical humped individuals. 

 Along with these abrupt transitions there usually occur, 

 however, a lesser number that are a little more gradual. 

 Individuals occur like saccates in all respects save that 

 they possess the inconspicuous dorsal hump; others are 

 small with the lateral and ventral (posterior) humps 



