12 THE SCOPE OF ZOOLOGY 



living things, it is true, do show some response to their environment — 

 a piece of steel placed in the sunlight will expand from the heat, but 

 this response is purely mechanical and certainly could not be considered 

 advantageous to the steel. It should be pointed out that responses of 

 living things under certain circumstances may turn out to their dis- 

 advantage. A moth responds to bright objects by flying toward them. 

 When these happen to be one of the many kinds of lights used by man, 

 the results may be disastrous for the moth. This tendency of moths 

 to fly toward bright objects probably has an advantage in a more 

 natural environment. It probably leads them to the light colored petals 

 of flowers at night where they may feed on the nectar, but this response 

 becomes a disadvantage in the blaze of lights that man has created. 



Long-term responses are slower in developing, but they last longer. 

 For instance, if you are exposed to unaccustomed amounts of sunlight, 

 you may be sunburned ; but regular exposure over a period of time will 

 cause the development of a heavier layer of pigment in the skin which 

 will protect you from the burning rays. The tan will remain for 

 some time after you have ceased to expose yourself. Mammals living in 

 cold regions develop a heavier coat of fur than those of the same species 

 that live in warm climates. Some people who raise mammals for their 

 fur have found that those raised in a cold climate usually develop a 

 heavier coat of fur than those of the same stock raised in a warmer 

 climate. 



5. Adaptation to Environment Through Natural Selection. All liv- 

 ing things slowly achieve inherited adaptations to their environment 

 which are passed from generation to generation. This is accomplished 

 by the process known as natural selection. Living things tend to pro- 

 duce more offspring than can survive. There are differences among 

 these offspring which are due to heredity ; those best adapted to the 

 environment will tend to have an advantage in survival over those not 

 so well adapted. As a consequence, there is a selection for the heredi- 

 tary factors which best fit the organisms for their environment. 



As an example, the desert rat which inhabits dry regions of the 

 western United States can live its entire lifetime without ever taking 

 a drink of water and with dry seeds as its only food. As one of the 

 adaptations to this environment it has developed a type of kidney which 

 greatly concentrates the urine and thus loses little water from its body 

 in this manner. This kidney is about four times as efficient as the 

 human kidney in this respect. Such an adaptation probably came about 

 through the selective elimination of those rats with less efficient kid- 

 neys — only those with the most efficient kidneys survived and trans- 

 mitted this characteristic to their descendants. 



