348 C. W. MITCHELL AND J. H, POWERS 



generation after generation, it is clear that nothing akin to a new 

 species is formed if sexual reproduction again brings the species 

 back to its original form. 



This seems at first sight to be the case. All the fertilized or 

 resting eggs hatch as individuals of the small saccate type, which 

 represents in general the primitive phylogenetic form of the 

 entire genus. The astonishing variations of the parthenogenetic 

 series, germinal though they are, thus seem to be wiped out by 

 sexual reproduction much as the somatic variations of the com- 

 plex metazoan are wiped out by the same process. The question 

 which we wish to answer is thus, whether this wiping out process 

 is complete; whether the fertilized egg is so different a thing 

 from the parthenogenetic egg; whether the entire variation of the 

 species, and doubtless of the genus as a whole, is but a play of the 

 environment upon parthenogenesis as such, or whether it is the 

 result of the forces fundamentally modifying the gametic consti- 

 tution of the species. 



It should be pointed out that the degree of inheritance through 

 the resting or fertilized egg must obviously be less than the usual 

 degree of inheritance through the parthenogenetic egg, since the 

 latter shows at least a strong tendency to maintain the species or 

 at least the race in question in a constant physiological and mor- 

 phological state. This the resting egg does not do; the first 

 generation of young hatching from it, is always morphologically 

 the same, from whatever type of the species the egg is derived. 

 There is still, however, the possibility that the young saccate 

 individuals derived from different eggs differ physiologically and 

 in their reproductive tendencies. 



Evidence h^ accumulated during our work on this rotifer 

 which tends to show that a parthenogenetic series of these small 

 animals, metazoa though they are, conducts itself very much as 

 does a series of protozoa derived from a single conjugant. Such a 

 series may thus be compared, with not a little truth, to the somatic 

 development of a larger metazoan resulting from the fertilized 

 egg. Now, just as in the metazoan, characters may crop out at 

 late stages and after a multitude of cell generations, which are 

 none the less determined by the inherent qualities of the ferti- 



