NATURAL SELECTION; SEXUAL SELECTION 69 



3. In other words, while natural selection furnishes the 

 motive force of change, other influences, extrinsic and intrinsic, 

 help to direct the channels in which life runs. It is necessary to 

 consider other causes for the great body of indifferent characters 

 or traits not produced by adaptation, and apparently not yield- 

 ing either advantage or disadvantage in the struggle for life. 



4. The formation of species of animals and plants through 

 natural selection finds an analogy in the formation of rivers 

 through gravitation. Gravitation is the motive power carrying 

 the waters from the uplands to the sea. The courses of streams 

 are determined by a number of minor influences acting in con- 

 currence with gravitation, the final result far more complex than 

 the single cause would produce. 



5. In like fashion, w r hile natural selection is the motive 

 element in descent or evolution, the total result is due to a 

 concurrence of causes, and is too complex to be explained by 

 natural selection, by the principle of utility, or the survival of 

 the fittest alone, and the varying effects must be ascribed to a 

 variety of causes. 



Certain minor traits, as color patterns, relative proportions 

 of parts, survive apparently without special utility, but because 

 these traits were borne by some ancestors or group of ancestors. 

 This has been called the Survival of the Existing. In making 

 up the fauna or flora of any region those organisms actually 

 present when the region is first stocked must leave their qual- 

 ities as an inheritance. If they cannot maintain themselves 

 their breed disappears. If they maintain themselves in iso- 

 lation their characters remain as those of a new species. In 

 hosts of cases, the survival of characters rests not on any 

 special usefulness or fitness, but on the fact that individuals 

 possessing these characters have inhabited or invaded a certain 

 area. The principle of utility explains survivals among com- 

 peting structures. It rarely accounts for qualities associated 

 with geographic distribution. The nature of the animals which 

 first colonize a district must determine what the future fauna 

 shall be. From their actual specific characters, largely traits 

 neither useful nor harmful, will be derived for the most part 

 the specific characters of their successors. 



It is not essential to the meadow lark that he should have 

 a black blotch on the breast or the outer tail feathers white. 



Yet all meadow larks have these marks, as all shore larks possess 

 6 



