24 EVOLUTION OF LIVING OEGANISMS. 



forth. But in the multicellular plants, Metaphyta, and 

 animals, Metazoa, where the cells form tissues and organs, 

 starting from primitive cells capable of performing 

 all the essential processes of life (irritable, metabolic, 

 growing, and reproductive) the cells become as differ- 

 entiation advances adept at performing certain functions, 

 but correspondingly incapable of carrying out others. 

 The more they become adapted in special directions, the 

 more they are apt to lose their other powers. So the 

 cells of the alimentary, nervous, muscular, and other 

 tissues of animals can only continue to live in association 

 with each other, supplying each other's wants. More- 

 over, the specialised cells of differentiated tissues can 

 usually only produce by division cells like unto them- 

 selves, and may lose the power of reproducing at all. 



Multicellular organisms all start from a cell in a primi- 

 tive undifferentiated condition, rich in unspecialised 

 protoplasm, capable of becoming adapted to perform any 

 of the necessary functions of life. Of such are built up 

 the early embryonic stages, and the actively growing 

 tissues of adults. But soon they become burdened with 

 the accumulated products of their own activities, their 

 potentialities are narrowed down, their irritability re- 

 stricted so as to respond only to special stimuli, their 

 capacity for regeneration exhausted. 



Speaking generally, the lower the organism the less 

 differentiated are its cells, and the greater are their 

 powers of growth and regeneration. A single cell of the 

 fresh-water alga Vaucheria or of the fungus Mucor will 

 grow into a new plant ; the propagation of plants by 

 cuttings is familiar. While the regenerating powers in 

 a man are restricted to the simple growth of tissues, the 

 healing of wounds and so forth, a newt or a crab will re- 

 generate a whole limb ; a worm deprived of its head or 

 tail will replace the missing parts ; a fresh- water polyp, 

 Hydra, may be cut into several pieces, each of which can 

 grow into a complete animal. Speaking generally, also, 



