Part I 



GENERAL CONCEPTS 



CHAPTER 2 



Protoplasm 



To DEFINE the field of zoology, or animal biology, it might seem a simple 

 task first to differentiate the living from the nonliving and then to sep- 

 arate the living into plants and animals. Yet each of these is quite 

 difficult to do sharply and clearly. Organisms such as cats, clams and 

 cicadas are clearly recognizable as animals, but sponges, for example, 

 were considered to be plants until well into the nineteenth century, and 

 there are single celled organisms which, even today, are called animals by 

 zoologists and plants by botanists. Even the line between living and non- 

 living is indistinct, for the viruses, too small to be seen with an ordinary 

 light microscope, can be considered either the simplest living things or 

 very complex, but nonliving, organic chemicals. 



Most biologists are agreed that all the varied phenomena of life 

 are ultimately explainable in terms of the same physical and chemical 

 principles which define nonliving systems. The idea that there are no 

 fundamental differences between living and nonliving things is some- 

 times called the mechanistic theory of life. An opposite view, widely 

 held by biologists until the present century, stated that some unique 

 force, not explainable in terms of physics and chemistry, is associated 

 with and controls life. The view that living and nonliving systems are 

 basically different and obey different laws is called vitalism. Many of 

 the phenomena that appeared to be so mysterious when first discovered 

 have subsequently proved to be understandable without invoking a 

 unique life force, and the vitalistic theory of life has lost supporters. 



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