AGENCIES, HABITAT, AND GROWTH 99 



social intercourse. They are the new agents which 

 convey the nourishing and sustaining agencies of so- 

 cial life ; they are the essential instruments for the dis- 

 tribution of diversified local products to the ultimate 

 growing points; they lengthen, ramify, and enlarge 

 with social growth, and society grows only so fast as 

 they better penetrate the great barriers of time and 

 space which separate man and nature, race and color, 

 the living and the dead, and establish between them the 

 life-giving channels of exchange. 



7. Growth by Multiplication (dissociation] and by 

 Combination (reunion or association] . The balancing 

 point in protoplasmic life is quickly attained within 

 the relatively narrow limits familiar to us in unicellular 

 life. When a cell reaches its maximum size and com- 

 plexity, no further growth in volume, or in organic de- 

 velopment, can take place except by subdivision, or 

 fragmentation; these fragments then grow up to the 

 same individual limitations as before, thereby produc- 

 ing more cells, distinct from one another, but of a sim- 

 ilar kind. This is what we call growth by multipli- 

 cation, or regeneration. But there is still a thircT 

 method of growth both in volume and complexity, due 

 to various amalgamations, or associations, of multi- 

 plying cells. Two or more cells may be completely 

 amalgamated with one another, as in the case of germ 

 cells ; or many generations of cells may remain in touch 

 with one another, still retaining their individuality, and 

 thus forming colonies, or multicellular bodies, com- 

 posed of millions of cooperating cells definitely ar- 

 ranged, it may be, into cooperating systems of tissues 

 and organs. Then these multicellular organisms may 

 multiply, and their offspring combine again and again, 



