ESSENTIALS OF ZOOLOGY 



CHAPTER I 

 INTEODUCTION 



The question, What is Life? is the greatest riddle in the biological 

 world. The term life is an abstraction with no objective reality ex- 

 cept as it is a phenomenon related to the activities of living units. 

 The following statement has been given and is probably as nearly a 

 definition as can be found : Life is a continuous series of reactions 

 in a complexly organized substance, by means of which the organi- 

 zation tends to adjust itself to a constantly varying environment. 

 Numerous attributes of living material may be given. Living mate- 

 rial has the ability to carry on active chemical reactions without 

 losing its body form. It is responsive to changes in the environ- 

 mental conditions; therefore, it is said to be adaptive. Living mate- 

 rial is able to sustain and reproduce itself under favorable conditions. 



Biology is a word derived from two Greek words, Mos, life, and 

 logos, discourse, and is the name universally applied to the study 

 of living organisms and life processes. Since living things fall 

 largely into two general categories, plants and animals, such a study 

 deals with the forms and phenomena exhibited by both. 



Nature is ever inviting investigation; her forces are in constant 

 operation about us, but she hides the truth. The biologist looks 

 upon himself as a seeker after truth, as one striving to get a glimpse 

 into the mysteries of life. As he succeeds in obtaining these 

 glimpses, he soon realizes the existence of certain fundamental 

 features common to the structure and function of all living forms. 

 He soon recognizes the oneness of all life, and himself as a part of 

 one great organic system, each unit of which has some relation to 

 the whole. A biological concept may rest upon observations, which 

 may be changed from day to day by the discovery of new facts, but 

 the biologist, like the chemist or physicist, is justified in holding to 

 a theory or hypothesis as long as it provides a true working basis 

 for further investigation. 



'^Trained and organized common sense" was the definition of 

 science given by Thomas H. Huxley, an eminent English biologist 



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