10 AN INTRODUCTION TO ZOOLOGY 



organism, and Prayer's aphorism " all life from life " is univer- 

 sally accepted. But though in the light of our present knowledge 

 all are willing to admit the truth of this aphorism, there are those 

 who hope that the advance of scientific knowledge may some time 

 in the future enable us to clear up the problem of the beginnings 

 of life which still remains unsolved. 



2. CHARACTERISTICS OF LIVING ORGANISMS 



All the objects that come within the range of human experi- 

 ence may be grouped into two classes, living and non-living. 

 Although our ideas concerning the origin of living organisms 

 cannot go beyond the assertion that they always come from other 

 living bodies, yet the characteristics that separate the two classes 

 can be stated in a very definite way. Living organisms are 

 characterized by (i) definite size, (2) definite form, (3) definite 

 elementary composition, (4) definite organization, (5) growth by 

 intussusception, (6) reproduction, and (7) irritability. 



(i) Size. If one were asked the question, Are organisms 

 uniform in size ? he would doubtless answer, No ! but on more 

 careful consideration it would become apparent that, although a 

 wide range of variation exists, yet the differences in size are con- 

 fined within rather definite limits. Certain parasites found in 

 human blood, which can only be seen with the highest powers of 

 the microscope, represent the smallest animals known ; the whale 

 is the largest. Although the discrepancy between these animals 

 is enormous, yet the range of variation illustrated by them has a 

 finite limit. On the other hand, the size variations of inorganic 

 substances are infinite ; for example, water is always recognizable 

 as such whether it exists in the form of minute particles of vapor 

 or reaches the dimensions of the Pacific Ocean. Although a 

 wide range of variation occurs among animals in general, the indi- 

 viduals of any given species are practically equal in size. In 

 many instances it is obvious that the size of a certain species 

 fits it for the conditions under which it lives. The whale could 

 not change places with one of the micro-organisms found in de- 



