46 NATURAL SCIENCE. July. 



connected story of recent progress, which will at least serve as a clue 

 through the very widely scattered literature of the past year or two. 



A good deal of attention has been paid to the phenomena of the 

 distribution of these creatures, which are so important to the student 

 of that branch of Biology. Apart from the notices of the occurrence 

 of new species the most recent summary of that side of the study of 

 the Oligochaeta is to be found in no. i. The difficulty which besets 

 every person who attempts to philosophise upon the facts of dis- 

 tribution is to disentangle the really indigenous from the newly im- 

 ported fauna. With earthworms this is apparently easier than with 

 many groups. One may fairly speak of a " Holarctic " region 

 embracing, of course, Europe, Northern Asia, and North America. 

 The most prevalent genus is the common earthworm of this country 

 (really consisting of many species), Lumhricus. There are somewhere 

 about sixty species of this genus, considered scnsu Into to include the very 

 closely allied A llolohophora and A llunis. Nearly all of these are common 

 to the Old and the New World ; or, rather, but few of the Lujiibricits 

 of America are different from those of Europe. Now it is probable, in 

 spite of the fact that this genus occurs most abundantly in gatherings 

 from almost every other part of the world, tropics as well as the 

 southern temperate latitudes, that the northern region is its true 

 home. Two principal facts support this contention, which are as 

 follows. The genus is most abundant in cultivated ground near the 

 coast in the extra-Holarctic parts of the world ; and, secondly, the species 

 found are invariably identical with those of the northern temperate 

 zone ; the true indigenous earthworm fauna of South America, for 

 example, has, as a rule, to be sought for outside the towns and their 

 immediate suburbs. Were the Liimhricus truly indigenous to these 

 parts of the world, one would fairly expect to meet with species different 

 to those of the north ; but one does not. Removing, therefore, this genus 

 from the indigenous fauna of any countries save those enumerated 

 above, it is easy to map out the globe into regions which, on the whole, 

 correspond with those of Mr. Sclater. The one possible addition 

 that might be made from this point of view to his six regions is an 

 Antarctic form ; but as I have created of that matter in a quite recent 

 number of this Journal, I leave it aside. Earthworms possess less 

 facilities for active migration than many animals ; a land-connection 

 seems to be nearly absolutely necessary to their wanderings. It is 

 perhaps otherwise with the aquatic Oligochaeta, which are often, in 

 correspondence with this, widely scattered. Vejdovsky (15) and I 

 myself (2) have found that the so-called cocoons of the small fresh- 

 water form ^olosonm, distinguished by the beautifully and variously- 

 coloured oil drops in the skin, are really chitinous cysts into which the 

 worm temporarily withdraws itself. This is an evident assistance to in- 

 voluntary migration, and it seems possible that some of the supposed 

 "species" of this genus which have been described from Africa and 

 America are identical with European species. The formation of 



