122 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



sion of the range of the Norway rat in the past century has had the 

 effect of reducing the range of the black rat. 



The capacity for active dispersal or passive transport (vagility) is 

 an important factor in the range of a form. A great range is common 

 among marine animals. Powerful swimmers are frequently world-wide 

 in range, like the tunny and other Scombridae, among fishes, or like 

 the toothed whales among marine mammals, Physeter, for example. 

 Among terrestrial animals it is usually the flying forms which are 

 widely distributed. Of the genera and families with a world-wide 

 distribution, the beetles and butterflies have the greatest number 

 among arthropods, the birds and bats among vertebrates. Species with 

 a world-wide range are confined to the birds among land vertebrates, 

 and principally to the butterflies among invertebrates. The thistle 

 butterfly {Vanessa car did) , which has such a range, is often seen 

 emigrating in swarms, and such emigrations often invade areas where 

 the species is unable to maintain itself, as in the British Isles. 1 The 

 milkweed butterfly is a wanderer of this type which has spread from 

 North America to the East Indies at a relatively recent date. It 

 arrived in the Tonga Islands in 1863, in Australia in 1871, and in 

 Celebes in 1873; it has reached the Azores in the opposite direction, 

 and is occasional in the British Isles. 2 The sphinx, Celerio lineata, 

 another powerful flier, has spread to all parts of the world. 3 A small 

 range is connected with lessened vagility, when related forms are 

 compared. The Satyridae and Zygaenidae, with a weak flight, are usu- 

 ally very local in their distribution, often confined to special habitats 

 or to specific localities. Such a distribution is unknown among the good 

 fliers, such as the Pieridae or Nymphalidae. 4 Flightless birds and birds 

 of weak flight (unless they are powerful runners) have a small range, 

 the flightless rails, for example. 



The facility with which some animals are transported may conduce 

 to an equally wide range. The inhabitants of transitory bodies of 

 water and of moss, which may be transported while in their resting 

 stages by the feet of birds or by winds, are frequently world-wide in 

 distribution. This is true, for example, of numerous rhizopods and 

 other protozoans; of threadworms, one of which, Bunonema richtersi, 

 occurs in the Black Forest, in Switzerland, in England, the Canary 

 Islands, and on Kerguelen and Possession islands; of Entomostraca"' 

 and the salt-crustacean Artemia salina; and of tardigrades Milnesium 

 tardigradum. 6 Conversely, only animals capable of active or passive 

 dispersal can inhabit such bodies of water. 



The animals which have been spread to all parts of the earth in 

 the train of civilized man are further examples of more or less acci- 



