20 CELLS AND THEIR PARTS 



consists of a collection of cells which have no intimate con- 

 nexion with one another, whose protoplasms do not inter- 

 mingle, and from which any cell can be separated without 

 harming the colony : here each cell lives independently of the 

 others. This is an example of a colony of the first order. 

 In the cell-colonies of the second order we find collections 

 of cells which at first are very much alike, but they cannot 

 live independently of the other cells that make up the colony. 

 Their flagella, if they have flagella, move in unison. If one 

 cell dies it must be replaced by another one. In colonies of 

 the second order there is no differentiation of function. Each 

 cell performs all the vital processes which are associated with 

 life, though some cells may be set aside to form eggs and 

 spermatozoa. But by far the greatest bulk of plants and 

 animals consists of numbers of cells vitally related to one 

 another, various functions being assigned to various sets of 

 cells. Their tissues consist of a collection of cells each of 

 which performs certain definite functions peculiar to that 

 tissue. Thus we get muscle tissue, nerve tissue, digestive 

 tissue, reproductive tissue, and so on. These form a colony 

 of the third order. 



Individuals 



These considerations raise the question as to what is an 

 individual, and that is a problem which is very difficult to 

 answer. We know quite well that the Prime Minister is an 

 individual, and the policeman round the corner is an individual, 

 so is a geranium, and so is a giraffe. But when we get down 

 through the lower plants and animals which reproduce by 

 budding the difficulty arises. On the whole, the cells that form 

 a colony of the first order, entirely independent of each other, 

 must be regarded as separate plants or animals and each cell 

 as an individual. But when we reach the colony of the second 

 order, where the cells interact and perform common functions, 

 here the Avhole colony must be looked on as an individual of 

 the same order as that of the higher plants and animals. 



