24 A PRIMER OF BIOLOGY 



many different kinds of tissues are bound together 

 into yet higher units. Thus, such an organ as the 

 liver consists not of glandular tissue alone, but of 

 connective, vascular, and nervous tissues as well, 

 and a leaf, in addition to the nutritive green cells of 

 which it is mainly composed, possesses also epidermal, 

 vascular and supporting tissues, arranged so as to 

 carry out most effectively the purposes for which they 

 were intended. Hence, as we study successively higher 

 grades of plants and animals, we come to recognise that 

 division of labour among cells is accompanied by 

 specialisation of structure ; that as the community 

 of cells becomes larger and larger, duties previously 

 carried out by all cells are now relegated to some 

 Division of cells only, and that these are, by their form, their 

 and UI contents or position, better adapted than others 



- f or the performance of these duties. We find also 

 structure, that there is a give-and-take among these cells, so 

 that the nutritive cells nourish not only themselves, 

 but all other cells requiring nourishment, while the 

 protective cells protect not only themselves but also 

 the nutritive cells which feed them. In a word, we 

 learn to appreciate one great generalisation in 

 biology, viz., that progressive specialisation of 

 structure is accompanied by physiological division 

 of labour. An instructive comparison may be drawn 

 between a unicellular and a multicellular plant or 

 animal on the one hand, and a human individual 

 and society on the other. Just as the unicellular 

 organism does everything for itself, so the isolated 

 human individual if, let us say, marooned on an 

 uninhabited island must be his own butcher, 

 tailor, shoemaker, grocer, builder and what not. 

 In a society, on the other hand, certain individuals 

 assume one duty to the community, others another, 



