402 Blr. William Batp Hardy [April 6, 



in waters draining different kinds of soil, and therefore chemically 

 different, the balance of sunshine and shade has been different, and 

 the wind capriciously sows these spores from north, south, east, and 

 west in the pot of hay tea. There they become active, and mate in 

 conjugation, but not fortuitously. Guided l)y chemiotaxis, unlikes 

 meet and fuse, just as do unlike cells when an (n'um and a spermato- 

 zoon fuse, and the fusion of a pair of unlike individuals results in 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. 



The problem of growth is this. Suppose we eliminate these 

 factors ; suppose we isolate a pure strain of paramoecia 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 diges- 

 tion and assimilation, on the one side ; while on the other it is con- 

 cerned with the limitations of heredity in moulding successive genera- 

 tions after the common type. Three workers have attacked it with 

 conspicuous success, Maupas,* Calkin,| and Woodworth, J and in each 

 case the experimental method was the same. 



Maupas was the first. He isolated individual paramoecia 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 

 promjjted by a remiirkable observation made by Prof. Loeb. lie 

 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 paramoecia ? So argued Prof. 



Arch. d. Zoul. Exp. 1889 (2) vii. t Arch. f. Protistenkunde, i. 1902. 

 X Journ. of Exp. Zool. ii. 1905. 



