THE LIFE CYCLE 353 



V. ARTIFICIAL DIVISION AND COMBINATION OF ORGANISMS. 



In the arts of men, artificial division for increase of 

 organisms asexually and artificial combination of the parts 

 of two organisms for the purpose of temporarily combining 

 their characters in one individual, have long been success- 

 fully practiced. The former is known in the gardener's art 

 as artificial propagation; the latter, as grafting. The parts 

 of an organism that are to grow up into separate whole 

 organisms must contain cells sufficiently undifferentiated 

 either to be able to redevelop the missing parts, or to re- 

 shape them out of pre-existing tissues. The parts of two 

 organisms that are to be organically joined together must 

 contain cells sufficiently plastic and formative to be able 

 to effect organic union between the conjoined parts. 



Regeneration.— The artificial propagation of the gardener 

 is called, when practiced by the zoologist, regeneration; and 

 this name embodies the essential phenomena involved — the 

 redevelopment of the missing parts of a piece of an organ- 

 ism. The slip cut from the top of an old geranium lacks 

 roots, and when placed in wet sand in a window box, it 

 develops a new root system out of the undifferentiated tis- 

 sues of its base, and does not proceed much further with 

 leaf development until roots are established. 



The capacity for regenerating missing parts varies much 

 in different organisms. It is very great in most plants, and 

 in many of the lower animals; but it is so poor in ourselves, 

 that after we reach adult life we may hardly replace a patch 

 of skin well enough to avoid a permanent scar. If the 

 tentacle of a hydra be cut off, another promptly grows 

 from the base of the old one. If the body be cut in two, two 

 perfect hydras develop from the parts, a new foot being 

 formed on the one and a new head on the other. Indeed, 

 the body may be cut into many pieces, and each piece that 



