326 STRUCTURE OF THE VERTEBRATES 



way been carried miles inland to fresh water lakes. Many cases 

 are known where mammals have been transported across straits 

 and large bodies of water on floating ice, logs, or tiny floating 

 islands of matted roots and earth which have broken from their 

 anchorage on the shore. Under these conditions a single pregnant 

 female would start a new colony, provided the climate and 

 other conditions were suitable. 



An inherent tendency toward roaming activity would cause 

 a marginal extension of range. The localized area occupied by 

 a specific group is gradually enlarged at the periphery. An 

 extension of the principle is a longer migration of a group. Al- 

 though there is a tendency among higher vertebrates to return 

 to the same location, which might militate against permanent 

 residence in the new environment, it is not unusual for animals 

 or groups of animals to wander off and become lost. 



According to Darwin the greatest cause of migration is the 

 struggle for existence. Survival of one group means death for 

 another, because the food supply of a region is soon exhausted. 

 Only a perfect balance between plants and animals, and a com- 

 plete limitation of the birth rate, could prevent this. As neither 

 has ever been found in nature, it follows that there is a constant 

 warfare between species, and individuals of the same species, 

 for a proper supply of food and a place to live. Certain individ- 

 uals would undoubtedly wander and start' new colonies in new 

 regions. 



Migration pathways are equally important, and always a 

 group would find obstacles in the way. Such natural obstacles are 

 barriers. The effectiveness of barriers depends upon the habits 

 and specializations of the animals involved. Mountain ranges 

 would not stop birds in their flight, but dry rocky slopes would 

 effectively prevent the migration of reptiles and amphibia. On 

 the other hand, mountains might aid migration if the range lay 

 north and south. To illustrate, urodele amphibia are unadapted 

 for extreme heat; but in southerly migrations these animals 

 would ascend the slopes where the mean temperature is lower, 

 and would go to higher altitudes as they moved to lower latitudes. 

 These conditions are known in such ranges as the Rockies, where 

 the urodeles are found in the higher valleys of INIexico, many 

 miles south of their normal range. The same mountain range 



