696 The Outline of Science 



periods of starvation by the simple procedure of living on them- 

 selves, gradually decreasing in size the while. As they get smaller, 

 they also become more active. This activity seems to be a real sign 

 of rejuvenescence, of renewed youth, for if they are alternately 

 fed and starved so as to keep them within certain limits of size, 

 they do not age, and persist, as Professor Child has shown, for as 

 long as the experimenter has patience to continue his experiment. 



Resting Stages 



Other animals have the extraordinary property of "living 

 backwards" in another way, of reverting to a more simple con- 

 dition by a process which, since it is the opposite of differentia- 

 tion, is styled de-differentiation. The Ascidian Clavellina, for 

 instance, although a sedentary animal, is of considerable com- 

 plexity, with gill-slits and heart, stomach and intestine, nervous 

 and reproductive systems. When it is placed in unfavourable 

 conditions, which yet are not sufficient to kill it, it shrinks, be- 

 comes more and more opaque, all the different kinds of cells of 

 which it is made become more and more alike, until it at last 

 comes to consist of a mere white lump, shapeless, and containing 

 nothing but a few rounded bags and a mass of loose cells to 

 represent all its original complexity. When placed in clean 

 water again, it develops anew, and becomes once more a normal 

 individual, of somewhat smaller size than before. These pro- 

 cesses of de-differentiation are of great importance in many uni- 

 cellular animals and in Bacteria; they lead to the formation of 

 resting stages in which the organisms can tide over hard times. 

 Further, since de-differentiation can be followed, apparently 

 over and over again, by fresh differentiation, the simple individ- 

 ual can be made to live an indefinite period by such means. 



These methods are not effective with higher animals. But 

 even in insects, and probably in all other cold-blooded creatures, 

 life may be greatly prolonged by low temperature. Professor 

 Loeb, for instance, found that while the life of the little American 



