NATURAL SELECTION 393 



3. All organisms vary, their variations including useful and 

 harmful, as well as indifferent characters. 



4. In the struggle for existence favorable variations are pre- 

 served and harmful eliminated, while indifferent variations may- 

 be incidentally influenced. 



5. As a second result of crowding some individuals may be forced 

 into other habitats involving the use of other characteristics, or 



6. Some may be forced into adjacent regions, in which variation 

 may be of immediate importance, either as a result of competition 

 or by geological disturbances. 



7. Any variation either isolated or emphasized by any of these 

 processes will be preserved through heredity and made a character 

 of the species or variety descended from the originally isolated 

 progenitors. 



8. Successive repetition of the process results in gradual change 

 leading to an ultimately wide separation from the parent 

 species. 



Examples of Natural Selection. Hypothetical. As an example 

 of the operation of this process, we may assume a case of a carniv- 

 orous species, of which ten thousand individuals live in a given 

 area. The prey of these animals may be chiefly an herbivorous 

 species, which, because of an epidemic disease or some other 

 factor, is reduced in numbers to a point where it can suffice for 

 the food supply of only one-half of the carnivorous species. 

 Granting that the sole defense of the herbivorous species is speed, 

 it is very probable that the five thousand carnivores best adapted 

 for speed will l)e the survivors; they alone would be able to repro- 

 duce, and the inherited characters which gave them superior speed 

 would become a quality of the subsequent generations. The 

 herbivores likewise would be able to survive only if swift enough 

 to escape, consequently they too would attain greater speed as 

 a fixed quality of the species. 



Wallace's Orchid. Wallace cites the case of a Madagascar 

 orchid {Angraecum sesquipedale) with an extremely long and deep 

 nectary. The flower is cross-fertilized only by long-tongued moths. 

 In reaching for the nectar the base of the proboscis comes into 

 contact with the anthers, and pollen is carried to the next flower. 

 Wallace's explanation of the case follows: "Now let us start from 

 the time when the nectary was only half its present length or 

 about six inches, and was chiefly fertilized by a species of moth 

 which appeared at the time of the plant's flowering, and whose 



