196 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



and our collections contain six undescribed forms. The abundance 

 of material is but little less important than the number of species 

 and subspecies represented, for the genus is one in which many of the 

 forms intergrade in away that can only be made to appear from study 

 of extensive collections. We wish to express to the United States 

 Commissioner of Fish and Fisheries and to the authorities of the 

 United States National Museum our hearty thanks for the privilege 

 of working upon these great collections, among the most extensive 

 and probably the most varied ever gathered. 



At the same time that work upon these collections has been going 

 forward, there have been under way studies by the senior author 

 upon the United States Bureau of Fisheries and United States Nat- 

 ional Museum collections of Salpidae, gathered in general from the 

 same waters as the collections of Pyrosoma. Among the Salpidae, 

 species are sharply distinct: In the genus Pyrosoma, on the other 

 hand, there is such intergradation that entirely confident demarca- 

 tion of species and subspecies is not possible. Intercrossing between 

 species is not indicated among the Salpidae. Among the Pyrosomata 

 it is altogether probable, cross fertilization seeming to be universal, 

 and different forms being known to be present together in the same 

 waters. Under these conditions it is probable that some of the forms 

 of Pyrosoma found are but transient forms, the genus being in flux, 

 interbreeding causing new combinations of characters to appear from 

 generation to generation. 



How shall such a genus be treated from the taxonomic point of 

 view ? Upon a strictly scientific definition of species every mutation, 

 however slight the divergence, if it be a true mutation, establishes a 

 new species with a new species mean around which is clustered a 

 whole group of conditions due to fluctuating variation. Whether 

 mutation is now occurring often in Pyrosoma we do not know. It 

 must have been frequent in the past, establishing a remarkable series 

 of divergent conditions in regard to many of the characters of the 

 organism, for we find such diversity today in these characters. 



Does each combination of conditions of the several characters, 

 which we find today, represent properly a species, or are such com- 

 binations, due to the mere shuffling of characters, not to be so regarded ? 

 The first appearance of a new character, or of a new condition of an 

 old character, arising through mutation, must be said, strictly con- 

 sidered, to produce a new species. Any attempt at demarcation of 

 species on any other basis, within a mutating and interbreeding group 

 of organisms, introduces too much of the personal judgment of the 

 student to be truly scientific. 



If into a group like Pyrosoma there should be introduced some influ- 

 ence preventing interbreeding, of course each combination of charac- 

 ters now present would be persistent and would represent a true 

 species, however similar some of these species might be to one another. 



