34 



LIFE: ITS BEGINNINGS AND NATURE 



those with the chromatin well centralized. 

 The next step in this long competitive path 

 would be the accumulation of more ma- 

 terial about the free-living "nucleus" to 

 make the cytoplasm, thereby forming the 

 first cell as we know it today. 



From this brief discussion it should be 

 obvious that it is quite impossible to de- 

 termine just what the first living thing was, 

 and this is exactly what one might expect. 

 By tracing living things backwards, the 

 inevitable end would be the inorganic 

 world; precisely when and where and how 

 the point of departure was established is 

 still a controversial question and will prob- 

 ably always be. Our biological studies have 

 progressed far enough today to give us an 

 inkling as to how it might have started, and 

 with that we must go on to an examination 

 of a few of the characteristics of living 

 things that exist on the earth today. 



THE NATURE OF LIFE 



There are certain criteria which our 

 study of living things has taught us to 

 associate with them. No doubt exists as to 

 whether or not a horse or rabbit is alive, 

 but without some knowledge of a tree one 

 might think that it is not alive, at least not 

 in the sense the rabbit or horse is. Even 

 the biologist becomes confused when he 

 studies such things as viruses which fail 

 to measure up in all respects to the living 

 things he is accustomed to investigate. 

 However, there are certain, rather well- 

 defined, characteristics that pertain to liv- 

 ing things alone, viruses excepted, which 

 are not encountered in the inanimate world. 

 1. Movement. Life is endowed with the 

 ability to move, and by that is meant 

 autonomous movement, the energy for 

 which comes from within (Fig. 2-11). To 

 be sure, water moves in a river, a stone 

 may roll down a hill, a car moves, but all 

 of this movement is due to forces acting 

 from the outside. The water in the river 

 bed flows, the stone rolls down hill, both 



because of the pull of gravity; the car 

 moves because a force from the engine is 

 applied to the wheels, causing a forward 

 movement of the vehicle. This type of 

 movement is quite different from that seen 

 in the rabbit scurrying through the thicket 

 with the dog close at its heels. In this case, 

 both are expending energy to move in 

 whatever direction each desires. Life im- 

 plies movement, life is dynamic. From the 

 gross movement of the entire organism to 

 the activity going on within each cell of 

 the body there is always change, always 

 movement. Such movements are linked 

 with living things and are not duplicated 

 anywhere in the inanimate world. 



2. Irritability. All organisms exist in an 

 external world which is spoken of as their 

 environment. With this world they are all 

 intimately associated, and it is impossible 

 to think of the organism without its environ- 

 ment. If living things are to profit by this 

 association, they must at all times be aware 

 of the nature of their immediate external 

 world. For this reason they are endowed 

 with the ability to sense its characteristics 

 and respond to them. The response may be 

 favorable, in which case the organism stays 

 in the environment. On the other hand, it 

 may be unfavorable and as a result the 

 organism moves out of the environment, 

 sometimes very rapidly as when a pin 

 makes contact with the skin of a small 

 boy (Fig. 2-12). Such responses play a 

 large part in the survival and ultimate suc- 

 cess of a race. 



An animal is equipped with an elaborate 

 set of sense organs that keep it in constant 

 contact with its external world; the func- 

 tioning of these organs make the difference 

 between life and death of the species. The 

 eyes are sensitive to light, the ears to 

 sounds, the nose to chemicals; all of these 

 assist the animal to orient itself in its en- 

 vironment, and to respond in such a way 

 as to permit its continuance as an individual 

 and as a race. 



3. Nutrition. In order that living things 



