6 INTRODUCTION 



cells. And although the unicellular plants and animals can 

 have no tissues or organs in this sense, yet since they perform 

 functions analogous to those of the higher forms of living 

 matter, we are correct in considering them organisms also. 



Thus we see that living matter exists in the form of indi- 

 viduals which are merely so many isolated particles ; the larg- 

 est of these particles in existence to-day are represented among 

 plants by the giant redwood trees of California, among animals 

 by some of the whales, and yet these largest masses of living 

 matter are microscopic as compared with many masses of inor- 

 ganic matter, such as mountains, lakes, and oceans ; living 

 matter never attains to such colossal proportions. And if we 

 consider further the total number of organisms inhabiting the 

 earth to-day, we shall soon see that by far the vast majority 

 are in truth microscopic ; a single drop of fluid may contain 

 more living bacteria than there are human beings in a large 

 city, and yet each bacterium is a complete and distinct indi- 

 vidual. 



Most organisms then are entirely invisible to the naked eye ; 

 the animals and plants which we can see form a very small 

 proportion of the individuals in existence ; presenting, as they 

 do, such a vast variety of form and structure, it is clearly quite 

 impossible for a single human being to become familiar with 

 them all. The number of animals in existence is inconceivably 

 great, but happily the number of different kinds of animals falls 

 within our human conception. Estimates concerning this num- 

 ber miist necessarily vary, they cannot be exact ; it has been 

 said that there are in the neighborhood of two million different 

 kinds of living animals, certainly a large number. But we find 

 that these different animals can all be assigned to two or three 

 large groups, the members of which all resemble one another in 

 certain respects and differ in others from all the members of the 

 other groups. These can further be subdivided into smaller 

 groups and the process continued until we have numbers which 

 can be handled with some facilitv. Thus bv a system of 

 classification we bring to light for the first time the fundamental 

 resemblances between various animals and in consequence their 

 relationships ; hence we see that classification is an absolute 

 essential to a knowledge of the science of zoology, it brings order 



