508 ESSENTIALS OF ZOOLOGY 



only a limited number of the increase in individuals can be sup- 

 ported in a particular habitat ; thus a struggle ensues with each in- 

 dividual attempting to secure the necessities of life. Not only is 

 there a struggle for food but also with many factors in the environ- 

 ment like climate, geographic changes, etc. 



Survival of the fittest was the outcome which Darwin saw resulting 

 from such a struggle. Those individuals which were best adapted to 

 the environment into which they were born have been the ones to 

 win out in the struggle and leave offspring for a future generation. 

 The inheritance of favorable or unfavorable pharacters influences very 

 strongly the success of the individual in maintaining itself. The sur- 

 vivors in any generation are those which inherit the most favorable 

 combination of variations. Many variations, both favorable and un- 

 favorable to the success of the individual, are hereditary. 



When changes in environment come and bring about new living 

 conditions, the animals in the particular habitat must either meet 

 these changes, be able to migrate, or perish. The standard for fitness 

 has changed under such circumstances, and animals with somewhat 

 different characters and adaptations may now be the ' ' fittest. ' ' The 

 individuals whose variations have brought them to most nearly fit 

 the requirements for life in their particular habitat will be the ones 

 most likely to obtain sufficient food supply and adequate provision 

 for reproduction to increase their population rapidly. As one group 

 is able to do this, it invariably reduces or perhaps entirely eliminates 

 other species in the locality. Evolutionary changes result from sur- 

 vival of the fittest, since those individuals which succeed have done 

 so because of an accumulation of favorable variations in each of suc- 

 cessive generations. Those individuals or races which have not been 

 as well adapted to conditions of the habitat have become inconse- 

 quential or extinct. The appearance of characters in an animal is a 

 matter of chance as far as the individual is concerned. Natural 

 selection may act as an eliminating agent and may determine whether 

 the character or trait shall survive after it appears. 



Mutation Theory. — Hugo de Vries is the name most prominently 

 connected with the origin of this theory. He was a Dutch botanist 

 and in 1886 found some evening primrose plants (Oenothera lamarki- 

 ana) which exhibited discontinuous variation or sudden appearance 

 of new characters. These sudden, sharp variations came to be known 

 as mutations. There are two types of variations which have been 



