192 Transactions of the Society. 



which you mention about their being adapted to certain temperatures, 

 but becoming gradually accustomed to much higher ones, is very 

 remarkable. It explains the existence of algae in hot springs. How 

 extremely interesting an examination under high powers on the spot, of 

 the mud of such springs would be." 



Shortly after this I brought my tentative and experimental work to a 

 close, and commenced the course of observations I shall here detail. 



I need not remind you that all biological changes must be slow. 

 Variations are constant, of that there can be no doubt; and under 

 domestication they are very palpably increased and conserved. 



But the smallness of every variation, as a rule, and the relative 

 fewness of the generations that come into existence, even of prolific 

 animals and plants during the working life of an observer, to say 

 nothing of the difficulties that would present themselves in other ways, 

 make anything like individual observation on visible forms under 

 experiment almost impossible and hopeless. 



Save for the useful and remarkable modifications efiected in animals 

 and plants under domestication, the great process of biological pro- 

 gression, made clear to us by Darwin, is essentially a secular one, and 

 is comparable to the vast secular processes of astronomy, such as the 

 precession of the equinoxes ; which, although we have observed but a 

 minute fraction of the complete cycle of movement, leaves us as certain 

 of what that cycle is as though we had traversed its immense circum- 

 ference under continuous observation. 



But this very fact, as in astronomy, so in biology, makes any 

 observed facts that may come within our reach, or be possible to our 

 laboratories, of even enhanced value. 



Now in the Infusoria — say the septic organisms — the cycle of life is 

 so relatively short, and the generations succeed each other so rapidly, 

 while the successive progenies can be so easily observed, that if we. 

 can devise apparatus and conditions which will enable us to institute 

 slow changes of environment, we should be able to observe critically 

 how far changes in the organisms led to responsive adaptations and 

 successive survival. At the same time it would be possible to closely 

 investigate the condition and appearance of the organisms themselves. 



To know the living forms under experiment, through all the changes 

 of their life-cycle, was therefore important ; and I chose such of the 

 monads whose life-history I had worked out, as were most easily 

 obtainable and most abundant. 



Every ingenious mind will have its own suggestion for the modi- 

 fication of the environment of such organisms ; bat after the tentative 

 work I had already done, and in view of the fact that at that time the 

 question of the influence of heat on this whole series of putrefactive 

 organisms was being eagerly discussed, I determined finally to make 

 cumulative increments of heat the means of adverse environment ; 

 I wanted therefore a delicate thermostat, that should be capable of 

 alteration at will to the temperature at which it should again become 

 static. 



After I had made a considerable number of preliminary experiments, 

 and had been aided by suggestions, especially from my friend Mr. Joseph 



