460 THE DISTRIBUTION OF ANIMALS 



stance, which were formerly abundant on the earth, but are now declin- 

 ing in numbers and may be facing extinction. These we often find 

 existing only in the isolated regions where they have temporarily escaped 

 the dangers which have exterminated those of their kind in other parts 

 of the world. Such species include the egg-laying mammals of Aus- 

 tralia, and the reptile, Sphenodon, which has a third eye on the top of its 

 head and is now found only in a few islands off the coast of New Zealand. 

 The extent of the range of animals is also due to the means of dis- 

 persal available to the species. Strong fliers, such as the ocean birds, 

 generally have a very extensive range. We have already learned how 

 the arctic tern breeds in northern Canada and winters in the antarctic 

 regions. Strong swimmers, such as the toothed whales, are often 

 world-wide in distribution. Eels which breed in the Sargasso Sea near 

 the Bermuda Islands migrate to the fresh-water rivers of several con- 

 tinents. Many protozoa are also world-wide in their distribution, be- 

 cause they often "hitch rides" in mud on the feet of migrating birds. 



Barriers to Dispersal 



Barriers of different kinds bar the way to animal dispersal. Let us 

 examine some of these with respect to particular animals. 



Geographical Barriers. Mountain ranges, deep valleys, oceans, and 

 land may serve as barriers. These are transient barriers in terms of 

 great geological eras, but can be very formidable barriers for consider- 

 able periods of time. The Rocky Mountains are high enough to block 

 the movement of many animals and, as a result, the animals on the At- 

 lantic Coast are very different from those on the Pacific Coast. The 

 Alleghenies, on the other hand, are usually less than a mile above sea 

 level at their highest peaks and there are many gaps through which 

 animals can migrate. As a result, life on both sides of these mountains 

 is very similar. They are high enough, however, to furnish an avenue 

 for the southern migration of both plants and animals from Canada 

 which extend their ranges southward as far as northern Georgia. On 

 the tops of the Great Smoky Mountains there is a fir-spruce zone in 

 which birds, typical of this zone in Canada, nest. 



Land is a big barrier to both marine and fresh-water fish. Before 

 the Panama Canal was built, the fish on the west coast of Panama were 

 very different from those on the east coast even though they lived only 

 a few miles apart. During the last forty years, however, a few have 

 managed to move from one ocean to the other through the canal and 

 are now established in a new ocean. Fresh-water fish are often confined 

 to a particular drainage system and cannot cross the bridge of land into 



