482 F. A. POTTS. 



that the American form of Cioiia intestinal is differs 

 markedly, at least in its physiology, from the Mediterranean 

 type species, and that, as is illustrated in plants, species which 

 differ but little from each other in external appearance may be 

 respectively easily capable of self-fertilisation and entirely 

 restricted to cross-fertilisation. 



The free-living nematodes easily lend themselves to an 

 investigation of the effects of continued self- fertilisation. 

 Maupas organised cultures for this purpose, taking great care 

 that the eight hermaphrodites chosen in each generation as 

 the parents of the next should in no case have come into 

 contact with mature males. AVith Khabditis elegans, the 

 period of experiment lasted from the beginning of December 

 to the end of June, and in these seven months fifty-two con- 

 secutive generations were reared. During the whole of this 

 time no decline in vigour or productivity could be ascribed to 

 the continuance of self-fertilisation. It is true that imme- 

 diately afterwards the race became extinct owing to the 

 onset of sterility, but the cause of this may well be traced to 

 a sudden rise of temperature in the month of June (Maupas, 

 p. 493). That this is the true explanation is indicated by 

 the fact that Ehabditis duthiersi, another hermaphrodite 

 species, which had only been isolated from the possibility of 

 cross-fertilisation for a few weeks, became sterile at exactly 

 the same time Avhen its cultures were subjected to the same 

 conditions. 



In my own researches Diplogaster maupasi has existed 

 in cultures with no possibility of a cross through twenty-five 

 generations, and that with not the slightest deterioration of 

 the strain. It is hoped that under temperature conditions 

 more equable than those of Maupas' laboratory at Algiers it 

 will be possible to prove that self-fertilisation may continue 

 through a longer period and larger number of generations 

 than was the ease in R. elegans.^ 



^ The cultures have now (June 21st, 1910) been carried over £oi-tj- 

 six generations without cross-fertilisation with no obsei'vable diminution 

 in fertility. 



