THE PHYSICAL BASIS OF LIFE 189 



a thorough reorganisation, a fresh make-up, of the living matter 

 of each. The continuance of the race depends upon the change 

 of environment, upon the alternation of periods of activity with 

 periods of dormancy, and upon the fusion of unlike individuals ; 

 and for these the sequence of natural phenomena, of summer 

 and winter, of sunshine and shade, provides. 



Now, the problem of growth is this. Suppose we eliminate 

 these factors ; suppose we isolate a pure strain of Paramcecia 

 and keep them abundantly supplied with food, will the race 

 continue to flourish and grow indefinitely, or will it attain old 

 age and die off? The problem is far-reaching. It touches the 

 simple questions of function, of digestion and assimilation, on 

 the one side ; while on the other it is concerned with the 

 limitations of heredity in moulding successive generations after 

 the common type. Three workers have attacked it with con- 

 spicuous success, Maupas, 1 Calkin, 2 and Woodworth, 3 and in 

 each case the experimental method was the same. 



Maupas was the first. He isolated individual Paramcecia 

 under normal and healthy conditions — namely, in hay infusion 

 containing the bacteria on which they feed, which was changed 

 daily. Each individual was the starting-point in a sequence of 

 generations, there being, on the average, two generations in 

 three days. The rate of division was recorded, and the records 

 furnished the basis for a curve of vitality. 



The experiment established two points, the first being the 

 presence of fluctuations of vitality of fairly regular character — 

 " rhythms," they have been called. The curve alternately rises 

 and falls, and each complete " rhythm " — a rise and a fall, that 

 is — lasts about a month. The second is that the curve, as a 

 whole, steadily falls, each successive rise in vitality is a little 

 less than its predecessor, each depression a little lower, until — 

 about the 170th generation — the period of old age, of senile 

 decay, is reached, and the race dies out. 



There the matter was allowed to rest, until fresh experiments 

 were prompted by a remarkable observation made by Prof. 

 Loeb. He found that the unfertilised eggs of sea-urchins could 

 be made to develop by immersing them for a few hours in 

 sea-water containing a higher percentage of salt than ordinary. 

 If the eggs could be artificially aroused, why not the senile 



1 Arch. d. Zool. Exp. 1889 (2) vii. 2 Arch.f. Protiste?ikitnde, i. 1902. 



3 Journ, of Exp. Zool. ii. 1905. 



