198 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



ditions here, as in Pyrosoma, suggest constant interbreeding between 

 numerous forms originally established by divergent mutation. In 

 all groups of this sort, which show such perfect intergrading, species 

 as used in taxonomy, must be purely conventional. Only in groups 

 where physiological, or other, isolation prevails can species dis- 

 tinctions of any considerable magnitude and of scientific value be 

 made. In these groups taxonomic systems can be real, can express 

 clearly observed natural conditions. In groups like Pyrosoma at- 

 lanticum the lines of demarcation into subdivisions must in part be 

 artificial. 



We have assumed in the foregoing that interbreeding between 

 divergent forms is prevalent among the Pyrosomas. The conditions 

 in the genus seem different from those in a genus which is merely 

 given to great mutation without interbreeding. Such a genus is 

 Opalina among the Protozoa Ciliata, a genus the senior author has 

 been studying at the same time that these studies of Pyrosoma have 

 been going forward. The species of Opalina are numerous and 

 there are species which intergrade between more divergent species, 

 but the distinctions here seem to be truly specific. For example 

 in several cases, in species so similar that one is at first doubtful if 

 they be distinct, the number of chromosomes in the nuclei of the 

 two forms is found to be different. This seems a conclusive dis- 

 tinction. In other cases the form of the mitotic figure is different 

 in species otherwise so similar as to be distinguished only with dif- 

 ficulty. The isolation of the several species of Opalina within their 

 often distinctive hosts, for all are parasitic, makes the possibility 

 of interbreeding seem slight. Among the Opalinae we have appar- 

 ently numerous species 1 truly independent, which have arisen 

 through a strong tendency to divergent mutation, and we do not seem 

 to have in this group any reshuffling of unit characters through 

 interbreeding. 



Salpa, Pyrosoma, and Opalina, therefore, present taxonomic con- 

 ditions very interesting to compare. Salpa shows very distinct 

 species which do not intergrade and are not interbreeding. Oimlina 

 presents species which in some cases very perfectly intergrade but 

 apparently do not interbreed, their intergrading being due to the 

 completeness of the response to the influences tending to produce 

 mutants. In Pyrosoma there has been abundant mutation in many 

 of the characters, and the conditions strongly suggest that at least 

 some of the different forms are freely interbreeding, causing a per- 

 mutation of the several qualities into almost all possible combina- 

 tions. It would be interesting to do for Pyrosoma what has been 

 done for Opalina and to test for some of the more similar forms the 



1 These considerations are based upon the study of a number of as yet undcscribed American forms, as 

 well as upon the well-known species. 



