AND ZOOGEOGRAPHY OF INDIAN OLIGOCH^ETA. 125 



obliged to leave their homes by illness, or by unfavourable con- 

 ditions such as the flooding of the burrows ; many worms, if 

 extracted from their holes, are unable to make new ones, and 

 must die. Some, however, certainly possess the power of active 

 wandering, as is shown by the numbers sometimes found under 

 heaps of manure. But it is obvious that the peopling of a 

 territory by earthworms through their own exertions can only 

 be very slow. 



Not only so, but they are limited in their wanderings by 

 desert tracts — some degree of moisture in the soil is essential. 

 Snow-covered mountain ranges are another obstruction. And 

 especially the sea limits them, the majority of earthworms being 

 quite unable to pass even a narrow arm of salt water. 



"We have to recognise, however, that not all the terrestrial 

 forms are so strictly limited in their means of dispersal as the 

 above would imply. A tree-trunk floating down stream, or 

 earth between the hoofs of cattle, may transport worms or their 

 cocoons. More important is the part that man has played ; 

 Lumbricids. natives of Europe, have been introduced all over the 

 world along trade routes; in W. Australia they are almost the 

 only earthworms to be found near the towns ; the indigenous 

 fauna is to be sought in the remoter parts of the country. One of 

 the commonest worms of the Punjab is Allolobophora caliginoms; 

 certain species of Pheretima have been carried round the globe, 

 far from the region where the genus is endemic. Small worms 

 are more likely to be carried in this way than larger ones ; and 

 small species of Dichogaster, an African genus, are common 

 throughout the Malay Archipelago, and not rare in India. 

 Botanical Gardens are obviously likely to be centres of dispersal 

 for such introd uced species in a new country. Records at Kew 

 and Hamburg leave no doubt of the reality and abundance of 

 these transfers through the agency of man. 



There are also, of course, differences in the powers of the 

 worms themselves. Some species seem to be able to travel more 

 widely than others, and more quickly, and to adapt themselves 

 to new surroundings and establish themselves more easily; and 

 it may thus happen that a species spreads over a large region 

 quite apart from human interference. It is not always possible 

 to distinguish between these cases and those of introduction bv 

 man ; and Michaelsen has adopted thp name peregrine for the 

 widely wandering species, whether they owe their diffusion to 

 man's agency or to their own unaided powers. 



For the purposes of Zoogeography, the distribution of fresh 

 water and littoral forms is of little or no importance; and the 

 same holds for the peregrine forms among the terrestrial group. 

 Thus, in discussions on the place of origin and past history of 

 the genera or larger groups, as well as in coming to conclusions 

 as to the past distribution of land and water, we are limited 

 for our facts to earthworms in the strict sense, and to those 

 among them that have a definite and limited range. But 



