vol. 2, ft. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 161 



Cyclosalpa, Brooksia, Apsteinia, Salpa (proper), Ritteria, Iasis, 

 Thetys, Thalia, Pegea, and Traustedtia. This seems more convenient, 

 in view of the long established use of the genus name Salpa. These 

 groups are far more worthy of emphasis as genera than are many 

 groups, in other families, which are recognized as genera. It has 

 not seemed worth while to place the species Tiexagona in a distinct 

 subgenus, though it diverges considerably from the other Ritterias. 

 The resemblance is probably more deserving of emphasis than the 

 divergence. Similarly the resemblance of the species virgula to the 

 other Cyclosalpas seems more worthy of emphasis than its consider- 

 able divergence. 



Treating as subspecies varieties and formae the forms polae, tubercu- 

 lata, aspera, bicaudata, and bicristata seems reasonable, for their close 

 similarity, respectively, to the species pinnata, maxima, fusijormis, 

 confederata, and multitentaculata seems much more worth empha- 

 sizing than does their divergence each from its respective species. 



Bicaudata and polae are clearly good subspecies. The tuberculate 

 variety of Salpa maxima is, so far as known, a sharply distinct 

 variety, but the form aspera, as noted, completely intergrades with S. 

 fusiformus. Of course it would be of interest to know from ob- 

 served breeding, in each of these cases, if the species ever "throws" 

 the subspecies as a sport. This, apparently, we are likely never to 

 know. There is no sufficient reason for supposing that this does 

 occur. We must take the forms as we find them and endeavor in 

 our taxonomy to make the clearest expression of probable rela- 

 tionship. 



SPECIATION. 



The very considerable diversity of the species within the family 

 Salpidae is correlated with the great specialization of the family for 

 its pelagic life. The two sets of phenomena are but different phases 

 of the fundamental physiological quality of plasticity. This plas- 

 ticity, as indicated in the later stages of the evolution of the family, 

 during the time of formation of the diverse species, seems due not to 

 sensitiveness in response to environmental influences, but more 

 probably to an inherent instability working itself out in many direc- 

 tions, without special relation to environmental influence. The 

 external conditions under which the several species live are remark- 

 ably uniform. Their divergence has not been in the nature of fitting 

 into different types of environment. It must have resulted almost 

 wholly from inherent qualities. 



One naturally asks if there is evidence that hybridization has had 

 a part in this trend toward divergent speciation in the family. Of 

 course, in the complete absence of paleontological evidence, we can 

 not say what may have been in the past, but there is no evidence of 



