ANIMAL LIFE. 3 



plants, divided up among the diverse organs. It can eat and move, it 

 reacts under external stimulus, and it can reproduce its kind. 



In the higher animals (Metazoa they are called, in contradistinction 

 to the Protozoa, or unicellular animals just alluded to) the bod}^ consists 

 of a multitude of cells, and as we rise in the scale of animal complexity 

 the functions of each cell are more and more circumscribed, one group 

 of cells performing one single function in the animal economy, while 

 another is similarly limited. With this increase in size and physiological 

 and anatomical differentiation occurs the formation of tissues. Thus 

 certain cells are sensory, other's digestive, and so on through the whole 

 category of animal phenomena, the corresponding groups of cells forming 

 nervous, digestive, excretory, and reproductive tissues. 



There is, however, a connection between the Protozoa and the more 

 complex animals, in that the latter always arise from a single cell — the 

 egg. This divides, and the result is two cells ; again, and we have four 

 cells, and so on, the product being finally an immense number. With 

 increase in the number of cells, differentiation goes hand in hand, out- 

 lining the various structures and organs of the adult. The studv of these 

 changes is known as embryology, — a study which is producing some 

 of the most profound conclusions with regard to the animal world. 



In order to be readily available and easily grasped, any group of facts 

 must be arranged and classified according to some natural or arbitrary 

 standard. With regard to animals and plants the same is true ; and 

 naturalists have consequently constructed systems of classification in 

 which every known animal has its place. The key to these arrange- 

 ments is a more or less evident similarity in structure or mode of growth 

 of the forms concerned, those which most closely resemble each other 

 being placed nearer or in a lower category, while those which have fewer 

 points in common form larger or higher groups. The case is partially 

 paralleled by the civil divisions found in our geographies, — a number of 

 towns occur in one county, a number of counties in one state, and several 

 states in a country. The parallel holds farther; for, as the boundaries 

 of the major and minor civil divisions are constantly changing, so with 

 the limits and arrangement of groups of animals : in the one case the 

 change is the result of conquest, purchase, or separation ; in the other, of 

 further studies into the secrets of nature. 



A single example will make plain the philosophy which embodies 

 classification. All of our domesticated cats have many points in common, 

 and every one recognizes a cat at once, no matter whether it be white, 

 maltese, or brindled. We say that the numerous individuals all belong 

 to one species. Now lions and tigers have many points of similarity to 



