Biology 695 



standing of the cancer problem, the general ideas drawn from the 

 fields of regeneration, of dominance and subordination of parts, 

 of the decrease of growth-power during development, and of the 

 struggle between the parts of an animal, provide us with the 

 point of view which may be the beginning of future progress. 



9 

 Old Age and Death 



When fission occurs, as in unicellular animals, there is in a 

 certain sense no death. There is at least no corpse. Individuals 

 appear and disappear, but barring accidents, the same substance 

 flows on through time in a stream of growth and fission. In 

 multicellular animals, however, the germ-cells alone constitute 

 this stream; the body persists for a time, but eventually, even if 

 it escapes all accidents, dies an inevitable, natural death. In 

 general, as we mount higher in the evolutionary scale, we find 

 that the individual body has a longer span of life allotted to it; 

 and further, this increased length of life is due much less to an 

 increased period of growth than to a prolongation of the adult 

 period. In lower animals growth is usually continuous, only 

 terminated by death. But in all the higher, it comes to an end 

 comparatively soon, and the prime of life is a period when change 

 is reduced to a minimum and the animal can continue applying 

 what it has learned to the business of life untroubled by deep- 

 seated physiological alterations within itself. 



In many lower animals, the changes that lead to ageing are 

 definitely reversible, and age can be kept at bay under certain 

 experimental conditions. In unicellular organisms, for instance, 

 increase of size seems to be the cause which brings about fission 

 and the disappearance of the individual. By cutting off a small 

 part of the animal each time that it approaches full size, it is 

 possible to keep it from dividing, apparently for an indefinite 

 time. Or again, Planarian worms, in addition to their extraor- 

 dinary powers of regeneration, are able to survive very long 



