Chap. I.] TISSUES AND ORGANS. 9 



In making a general survey of animals we find 

 that the lowest consist only of simple cells ; later on, 

 the cells are found not to live an independent 

 existence, but to be associated one with another, and 

 different groups of cells are seen to be differentiated 

 in various ways. The result of this is that sets of 

 cells come to have different characters (some are 

 contractile, others irritable, and so on), and these 

 different sets are what are known as tissues ; 

 secondly, we observe that these tissues become 

 connected with one another in different proportions 

 and relations, so as to give rise to those parts of the 

 adult which take on particular duties, and are known 

 as organs. 



Looked at in a general way, and without taking 

 any notice of exceptional cases, we observe that 

 there are tissues in an animal which are not found in 

 a plant ; these, which are distinguished as the animal 

 tissues, are such as have a relation to movement or 

 sensation ; in other words, the muscles and nerves are 

 animal tissues. On the other hand, plants preserve, 

 protect, and sustain themselves, and the corresponding 

 tissues in animals are always spoken of as the 

 vegetative ; .of these we may find convenient 

 examples in that outer layer of the body which is 

 spoken of as epithelium, or that supporting tissue 

 which is known as bone. 



The classification of organs is a little more complex, 

 but it will be convenient to give it now, so that time 

 and space may be saved in the future. 



In the first place it is clear that the vegetative 

 functions fall under three great heads ; an animal has 

 to care for itself, to adapt itself to or move through 

 its surroundings, and to reproduce its kind. And, in 

 the second place, it is just as obvious that it has to 

 perceive what is going on around it, and to act 

 accordingly. We have, then : 



