CHAPTER LXXXIII 

 EVOLUTION 



485. All things change. We understand that we live in 

 a world of change, that in fact all of our experience, all of our 

 life, has to do with these changes. All theoretical studies are 

 concerned with changes ; and all practical studies agriculture 

 and medicine, engineering and statesmanship are concerned 

 with three sets of problems. These are 



1 . How can we caiise desirable changes to take place ? 



2. How can we prevent imdesirable changes from taking 

 place ? 



3. Hoiv can we best meet the unavoidable changes? 



486. Cyclic changes. Many of the changes that go on about 

 us are of a cyclic nature ; that is, they keep on repeating them- 

 selves. For example, the day gives way to night, but the night 

 gives way to day again, and so on indefinitely. 



Our seasons illustrate cyclic changes. To a mosquito (an 

 individual adult) weather may mean a continuous, progressive 

 change from warm to cold, resulting in death and ending 

 everything. 



The seasonal changes show us not only cyclic variations in 

 temperature, moisture, etc. but cyclic changes in the organic 

 world. Eggs hatch, individuals develop to maturity, reproduce 

 themselves, and die. But the following year we see a repetition 

 of the same life histories, and so on, generation after genera- 

 tion. With some species of animals the cycle extends over 

 many years, but the point is that, however far-reaching the indi- 

 vidual development may be, it does not go on forever ; it comes 

 to a close and is replaced by others that go through exactly the 

 same kinds of stages. In other words, life forms or life stages 



463 



