528 GENERAL CONSIDERATIONS 



paths of migration have been assumed. An example of such a land mass 

 is the hypothetical land named Lemuria, supposed formerly to have con- 

 nected India and Madagascar; this would account for the fact that there 

 are many types common to these two regions. The former existence of a 

 continent connected to both Australia and South America, known as 

 Antarctica, has also been suggested as a means by which related types 

 now found on those continents may at one time have been distributed 

 over a continuous area. 



571. Place of Origin. — It is usually assumed that each species of 

 animal must have originated in one particular place on the surface of 

 the earth, from which locality it has been dispersed throughout its range. 

 It is also generally assumed that closely related species have had a com- 

 mon origin and have been produced by modification of the type during 

 its dispersal. 



572. Dispersal of Animals.— Assuming a certain species or type of 

 animal to have originated in a certain place, its dispersal, if it is successful, 

 is inevitable; increasing numbers and the competition for food will of 

 themselves cause the individuals to spread over a constantly widening 

 area. Other factors which tend to cause animals to disperse are the 

 search for favorable places in which to rear their young and the safety 

 which isolation gives. Animals may, by changes in the environment 

 which make it untenable, be forced to move from their original home. 

 The following factors favor the wide dispersal of animals: (1) length of time 

 during which dispersal has taken place ; (2) uniformity of chmatic condi- 

 tions over a wide area; (3) continuity of habitat; (4) transportation by 

 water currents, floating objects, and wind; (5) attachment to the bodies 

 of other animals which move about; (6) human agencies. While a 

 criterion to be applied with great caution, there are some grounds for 

 the assumption that widely distributed types have had a long time dur- 

 ing which to acquire wide dispersal and are, therefore, older types than 

 those of a more restricted distribution. Most of the factors need no 

 explanation. It may be related, however, as an example of dispersal 

 by other animals, that on one occasion a blue-winged teal, shot near 

 Lincoln, Nebraska, was found to have in a mass of mud on one foot 

 five Uving crustaceans, Hyallela dentata (Say), two of which were females 

 with eggs. The introduction of foreign types of animals into any region 

 is being made constantly more easy by the development of international 

 systems of transportation and the freedom of commercial intercourse. 

 The dispersal of animals is sometimes called migration, though it should 

 be carefully distinguished from periodic migration (Sec. 575). 



573. Factors Hindering Dispersal. — Many conditions act as barriers 

 to the dispersal of animals. (1) Geographic barriers, such as mountains, 

 large bodies of water, and deserts, put a check to the dispersal of many 

 types. Open areas are barriers to woodland forms, while the forest is a 



