GEOGRAPHICAL DISTRIBUTION 37 I 



which live indifferently on land and in fresh water ; and even some 

 habitually terrestrial species can be kept in water for many weeks 

 with impunity. A desert, on the other hand, is a complete barrier ; 

 the animals are absolutely dependent upon moisture, and though 

 in dry weather the worms of tropical countries bury themselves 

 deep in the soil, and even make temporary cysts by the aid of their 

 mucous secretions, this would be of no avail except in countries 

 where there were at least occasional spells of wet weather. 



The range of the existing genera and species is quite in 

 keeping with the suggestions and facts already put forward. But 

 in considering them we must first of all eliminate the direct 

 influence of man. Every one who studies this group of animals 

 knows perfectly well that importations of plants frequently con- 

 tain accidentally-included earthworms; and there are other ways 

 in which the transference of species from one country to another 

 could be effected by man. There are various considerations which 

 enable us to form a fair opinion as to the probability of a given 

 species being really indigenous or imported. Oceanic islands 

 afford one test. There are species of earthworms known from a 

 good many, but with a few exceptions they are the same species 

 as those which occur on the nearest mainland ; in those cases 

 where it is supposed that the animal inhabitants have reached an 

 oceanic island by natural means of transit, it is a rule that the 

 species are different, and even the genera are frequently different. 

 That the bulk of them are the same seems to argue either 

 frequent natural communication with the mainland or a great 

 stability on the part of the species themselves. It is more 

 probable that the identity is in this case to be ascribed to 

 accidental transference. 



Another argument comes from the distribution of the family 

 Lumbricidae. This family forms the bulk of the earthworms of 

 the European and North American continents. But they are 

 also found all over the world. With one or two exceptions, such 

 as Allolohopliora moebii, from Madeira, the extra-north-temperate 

 species are identical with those found within that region. Now, 

 if the migration had been by natural means there would surely in 

 the lapse of time been some differentiation of species. Further- 

 more, Dr. Michaelsen has pointed out that in South America the 

 presumably European forms (i.e. Lumbricus and Allolohopliora) are 

 found upon the coast and in cultivated ground ; it is inland that 



