284 AN INTRODUCTION TO ZOOLOGY 



influence their life activities, so that we are able to see very little 

 if any increase, but even a possible decrease, in the number of 

 birds that greet us with the coming of each spring. 



These animals are not peculiar in any way, since any species 

 that might be selected would soon cover the earth with its de- 

 scendants if allowed to reproduce unchecked. In most cases the 

 number of individuals of a species is practically constant from 

 year to year. This is the result of the struggle for existence. 

 An animal must contend with its surroundings; Euglena must 

 have light to carry on its life processes; and the honeybee must 

 store up provisions and make its hive secure in order to survive 

 the winter. 



The habitat of a species is never large enough to accommodate 

 all the individuals of a species, and an animal is forced to battle 

 for life against others of its kind. The stronger, other things 

 being equal, always wins in the struggle, in other words, the fittest 

 survives. For example, in poor seasons for gathering honey, 

 bees from one hive attack those of another, and, in many cases, 

 carry off the pollen and honey, leaving their victims to starve. 

 Other battles occur between individuals of different species. 

 Whatever the cause of the struggle, it is always the weakest that 

 is vanquished. The strongest individuals of a species live on 

 and keep up the vigor of the race, this being one of the advan- 

 tages of the elimination of the unfit. 



(6) Heredity 



Heredity has been defined (p. 79) as the " resemblance of child 

 to parent." " A character may be said to be inherited when it 

 always, in one generation after another, is one of the characters of 

 the species, ..." (269, p. 15). The characters of an organism 

 depend on the nature of its surroundings, especially during de- 

 velopment, and on the constitution of the part of the parent from 

 which it grew. In Metazoons this may be a bud, as in Hydra, 

 or an unfertilized egg cell, as in the drone bee, but is usually an 

 egg fertilized by a sperm. 



