GEOGRAPHIC DISTRIBUTION 309 



The timing of the changes of species and the changes of the earth 

 must have had important consequences for hving things. When a 

 group of animals experienced the genetic changes which might lead to 

 the formation of a new species, any changes of the land or climate 

 occurring at the same time and in the same region could spell the differ- 

 ence between survival and destruction of the new group. When a region 

 of the earth was undergoing a physical (perhaps climatic) change, any 

 genetic change going on in a few individuals could decide whether any 

 members of their species would survive the changes of environment. 

 For the sake of emphasis these changes are described as sudden and 

 radical; actually they have been very gradual. Interplay of the physical 

 forces of the environment and the genetic forces of animal or plant life 

 must have been crucial in the guidance of evolution, and in the deter- 

 mination of the location of resulting species and higher groups. Let us 

 turn to some of the facts of distribution, to see how they fit into the 

 general scheme just outlined. 



Position of Ranges. — It is easily observed that species, families, 

 orders, etc., have their characteristic places on the earth. With the 

 exception of closely interdependent species, such as parasite and host, 

 probably no two species have exactly the same range. The musk ox is 

 arctic; the nine-banded armadillo ranges from Texas to South America; 

 the North American alligator exists only in the extreme southeastern 

 part of the United States. Among such vastly different groups, widely 

 separated ranges do not occasion any comment. Within a single genus, 

 however, the several species have their distinct areas. For example, in 

 the genus of spadefoot toads, Scaphiopus couchii extends from Texas to 

 Arizona, and into northern Mexico, including Lower California; S. ham- 

 mondii ranges from Montana to Mexico and west to the Pacific states; 

 8. holbrookii holbrookii occurs along the Atlantic from Massachusetts 

 to Florida, and west to Louisiana, Texas and Arkansas; S. holbrookii 

 albus is only in the Florida Keys, or possibly also the extreme tip of 

 Florida; and S. hurterii is found only in Texas. 



The location of a species range depends primarily on where the 

 species started. The present range must usually be around or near the 

 point of origin. Looking backward, one sees the ''center of dispersal" 

 of a species as some point in or near its present range. Most species 

 have not lived long enough to have traveled far. Very old species, 

 however, especially those which survive in only a few individuals, may 

 be far from their places of origin. Such old and nearly extinct species 

 may usually be recognized as such because no closely related species are 

 anywhere near them. Species consisting of few individuals because they 

 are very young are, on the contrary, surrounded by very similar types. 

 Remnants of very old species are spoken of as relicts. Examples are the 



