2 THE SMALLEST LIVING THINGS 



The Growth of Knowledge 



Biology, briefly defined, is the science of life, or the knowl- 

 edge of living things. Man has always been interested in the 

 subject as offering some possibility of an interpretation of him- 

 self, his mechanisms, and his origin. In pursuit of this knowledge 

 he has searched the remote parts of the earth; he has dug deep 

 into its solid substance and has explored the depths of the ocean. 

 He has studied and dissected every new type of living thing that 

 has been found; he has compared its structures and its functions 

 with those of organisms which were already known; he has 

 given each such new type two names by which it may be known 

 and identified; and he has arranged it, with other similar organ- 

 isms, into an orderly system of classification. 



The Thoroughness of Biological Research 



It is perfectly safe to state that no one knows exactly how 

 many animals and plants have been thus studied and classified; 

 various estimates, running into millions, have been made. Not 

 only the obvious structures which form the basis of the study of 

 anatomy have been studied and compared, but the finest details 

 of structures entering into the make-up of living substance have 

 been analyzed with equal care, and the sciences of histology and 

 cytology* have been the result. 



Not only structures but the activities or functions of living 

 organisms have been followed out with equal painstaking thor- 

 oughness. Here again the more obvious functions were the first 

 to be known and it was soon demonstrated that all animals and 

 plants respire, requiring oxygen for the purpose, and giving off 

 carbon dioxide as a result, that oxidation is the underlying phe- 

 nomenon Of all animal activities, and that waste matters result 

 which must be disposed of. It has long been known that such 

 activities involve the breakdown of living substance and that the 

 loss is made good by the addition of food substances. The two 

 series of activities — waste and repair — are comprised in the 

 biological term metabolism, destructive metabolism comprising 



* Histology and cytology are two branches of biology, both of which deal with 

 cells ; histology treats of cells chiefly as the elements constituting tissues, while 

 cytology treats of the individual cell with reference to its structure, function, 

 multiplication, and life history. 



