34 NATURAL HISTORY 



Overpopulation and Its Results 



One of the I'aclors in dcteniiiiiiug tlie .spread and distribution of 

 organisms is overpopulation. An annual plant, for example, pro- 

 ducing only two seeds a year, which is far below the actual number, 

 and always developing these into mature plants, in only twenty-one 

 years would have 1,048,576 descendants. A pair of common house- 

 flies which usually produces eggs six times a year, each batch con- 

 taining 150 to 200 eggs, with the young flies beginning in turn to 

 lay eggs in about fourteen days after hatching and repeating the life 

 cycle, might, it is calculated, beginning to breed in April, if all the 

 eggs were hatched and no individuals died, give rise to 191,010,000,- 

 000,000,000,000 descendants by the end of August. However, each 

 species, year in and year out, tends to remain about stationary in 

 number. Indeed, many species are actually disappearing. The 

 reasons for this check of potential populations are found in lack of 

 adequate food supply, lack of favorable breeding conditions, and in 

 the fact that many animals and plants become food for others. 



The Shifting World of Organisms 



There is no doubt that desire for food furnishes the greatest urge 

 to locomotion and exploration in animals. Dr. Crothers once said 

 in one of his essays that the "haps and mis-haps of the hungry make 

 up natural history." Indirectly there is the same necessity for food 

 on the part of plants, but here the urge is expressed not so much in 

 locomotion as in a struggle for position with reference to light, which 

 is essential to every green plant in the manufacture of its own food. 



Changing environmental conditions may force the movements of 

 organisms and produce faunal and floral repopulations. For example, 

 it is known that drifting coconuts frequently float long distances and 

 grow into trees upon some distant shore. A recent cataclysm of 

 nature has given us an opportunity to see the repopulation of a 

 devastated area taking place. In 1883, the volcanic island of Kra- 

 katao was literally blown to pieces by a series of terrific explosions 

 that destroyed every living thing on the island. Less than three 

 years after the volcano became quiescent, a Dutch botanist visiting 

 the island found the ash which covered its surface completely car- 

 peted with a layer of bacteria, diatoms, and primitive blue-green algae. 

 Here and there ferns were found, along with several kinds of mosses. 

 There were even a few flowering plants, but no trees or shrubs. In 



