AND ZOOGEOGRAPHY OF INDIAN OLIGOCH^TA. 131 



variability that would link it on to an existing species. The 

 number of described '• forms " and varieties is therefore large. 

 Sometimes, as in the case of Foiitodrihts, critical examination 

 and the increase of knowledge results in the union of a Avhole 

 series of species under a single name. 



Not only are transitions between species common, but the 

 same is true for genera. In speaking of the Megascolecinje in 

 the previous article it has been noted that literally all stages in 

 the passage from the lumbricinetothe perichsetine arrangement of 

 setse, from the meganepliridial to the microaephridial condition, 

 and from the tubular to the racemose prostates are met with. 

 The same holds for other characters which have been used as 

 generic distinctions, e. g. the well-developed or the vestigial 

 gizzard, the degree of approximation of the genital apertures, etc. 

 The consequent difficulty of separating genera has led Michaelsen 

 to fuse a number of genera, with, I think, a great saci'ifice of 

 convenience. 



In speaking of the S. Indian earthworm fauna, I have noted 

 (19) that the 2,qwVl& Megascolex seems to have "recently inider- 

 gone a notable blossoming forth, with the production of a large 

 number of forms a)id intermediate forms, and that in consequence 

 it is extremely difficult to separate species fiom varieties, and 

 varieties from examples of individual variability. The (Indian) 

 range of Megasaolex is of very limited extent, yet the number of 

 species is extraordinarily large ; and still every collector, wherever 

 he chooses to explore, brings back numerous novelties," Of the 

 sa,me kind is the discovery of what I have called a "nest ''of 

 related species of Brawida in the Chittagong district (20), and of 

 species of Ilojjlochcetella in a limited region of Westein India 

 {20). The same blossoming forth is seen in the Lumbricidse, 

 where the distinction of species, and especially of genera, is 

 notoriously difficult ; genus passes into genus — often into more 

 than one genus — and the same kinds of changes appear to be in 

 progress in different parts of the tree, to such an extent that the 

 confusion is almost inextricable. There is thus an appearance of 

 incomplete differentiation, and a lack of that fixity and extinc- 

 tion of intermediate forms which we are accustomed to associate 

 with old established groups. 



Add to this the mei'O length of the line of descent from the 

 supposed late Secondary ancestral eai'thworms to such forms as 

 Mejascolex and Pheretima, the latter portion of which has Ijeen 

 traced in the previous article. It seems highly improbable that 

 in such a vigorous group, and one so capable of adaptation to 

 new environments, the differentiation of genera, should have 

 ■ceased soon after its first rise — so long ago, say, as the Eocene. 



(!i) The Question of Land- Bridges In general. 



That the outlines of land and sea have changed during geo- 

 logical time is of course universally admitted ; but as to how great 



9* 



