i88 ELEMENTS OF SCIENCE 



different tissues) and each "system" (consisting of various 

 organs), the harmony of all the " systems " resulting in 

 the life of the creature whereof such " systems " are com- 

 ponent parts. 



Certain lowly organisms consist but of a single cell, 

 and are therefore spoken of as unicellular. 



We know that an animal (e.g., the cat) eats, digests, 

 and so nourishes itself, circulates its blood, breathes, 

 forms (i.e., secretes) saliva, bile, &c. ; feels, moves to and 

 fro, and may become the parent of another generation. 

 These various life activities, or functions, are respectively 

 known as alimentation, nutrition, circulation, respiration, 

 secretion, sensation, locomotion, and generation. Every 

 one knows also that plants and animals grow, as also 

 that plants generally spring from seeds, and birds from 

 eggs. 



The study of that particular growth which takes 

 place in a plant from the seed, and in the bird from 

 the formation of the egg, is called embryology. 



But animals and plants have very definite relations 

 with space and time. Monkeys and armadillos do not 

 exist in a wild state in England, and kangaroos are 

 found nowhere, naturally, save in the Australian region ; 

 nor are cacti, which are so common in Mexico, found 

 wild in Scotland. There is, then, a science of the 

 geography of organisms. They have also definite 

 relations to past time. A multitude of animals and 

 plants which existed in Eocene* times do not live now, 

 and this is still more the case with the reptiles whose 

 remains are found fossil in the secondary strata, while it 

 is clear that many animals which now live did not do 

 so in those earlier periods. Thus, organisms have 



* See ante, p. 169, 



