886 TEXTBOOK OF ZOOLOGY 



be ninety or a hundred years of age and averages only six progeny, 

 could soon occupy all of the standing room on the face of the earth. 

 Beginning with one pair of elephants and providing every individual 

 lived and reproduced even at the average slow rate mentioned, 

 19,000,000 individuals would be produced in 750 years. If every ele- 

 phant alive today were to enter into a program like that, both food 

 and space would become quite scarce before many generations. How- 

 ever, this doesn't happen on a large scale. All plants and animals 

 tend to produce more offspring than can ever reach maturity and re- 

 produce. 



The Struggle for Existence is ever present because there are more 

 individuals produced than the habitat will support. The two most 

 fundamental needs for which organisms struggle are: (1) food and 

 (2) opportunity to reproduce and rear young. Of these two, the 

 struggle for food is very immediate and the food supply is an im- 

 portant limiting factor on population from season to season. Since 

 the food supply, on the average, remains quite constant, it is evident 

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

 ported in a particular habitat. 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 environment 

 like climate, geographic changes, etc. 



Survival of the Fittest was the outcome which Darwin saw result- 

 ing 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 characters influences very 

 strongly the success of the individual in maintaining itself. The 

 survivors in any generation are those which inherit the most favorable 

 combination of variations. Many variations, both favorable and 

 unfavorable 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 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 rapidly increase their population. As one group 



