596 General and Applied Biology 



result may be extermination or an attempt to continue its migration. 

 This is an illustration of what is frequently known as a biologic barrier. 

 Much of this adaptive ability is due to inherent, inherited properties of 

 the protoplasm of each particular animal. 



The following methods of dispersal are rather common in the animal 

 world. ( 1 ) Driftwood may transport animals for great distances. Wil- 

 liam Beebe on his Arcturus voyage observed fifty-four species of marine 

 fisheSj worms, and crabs on one floating log. (2) Ships in their travels 

 from port to port may transport various types of organisms. How many 

 rats have had free transportation from one port to another can never 

 be known. (3) Water and floods may mechanically transport organisms, 

 drive them from their original habitats, or change the food supplies suf- 

 ficiently that dispersal will be necessary. The presence of desirable water 

 supplies for consumption during migration may determine the final and 

 future habitation. (4) Aquatic animals may transport other animals on 

 their bodies or within their bodies. The larvae of clams may be carried 

 on the gills and fins of fishes. Many aquatic parasites are dispersed by 

 aquatic organisms. (5) Terrestrial animals may transport other animals 

 on the exterior or interior of their bodies. Birds may carry eggs, larvae, 

 pupae, or adults of smaller animals, especially during migration. (6) 

 Winds may direct the course of certain animals or may blow objects to 

 which certain types are attached. (7) Glaciers may cause animal migra- 

 tions by actually transporting them or by changing the temperature or 

 food supply. (8) Man, either knowingly or unknowingly, aids in animal 

 dispersal through the means of automobiles, airplanes, boats, and trains. 

 A "horned toad" was transported from Texas to Springfield, Ohio, by a 

 circus train, although after its arrival it found the rigors of the city too 

 great. This was no fault of the method of migration. English sparrows 

 were transported from the East to the West in returning empty grain cars. 

 This method was successful for the rather friendly sparrow but would not 

 have been used by the more timid bluejay which would hesitate to fre- 

 quent the empty grain cars in the East. 



The principle of discontinuous distribution is illustrated by the presence 

 of the same species of animal in two widely separated regions, in which 

 case it is usually concluded that the distribution of that species was once 

 continuous between the present regions. For example, the tapirs today 

 inhabit Central and South America, southern Asia, and the Malay archi- 

 pelago only. In the Pliocene epoch of the past (Figs. 320 to 322), tapirs 

 were distributed over nearly all of North America, Europe, and northern 



