ON INFUSORIA. 119 



temperature until they reached the maximum of rapidity, which 

 varies indifferent species, at the temperature of 30° centigrade. If 

 the heat increased beyond that point, the rapidity of the con- 

 tractions diminished, while at the same time that of the ciliary 

 movements increased ; if, finally, a temperature of 35° was 

 attained, a striking difference was seen in the reaction of two 

 kinds of cilia ; those which effect the rotatory movement move 

 without being in any way affected by the animal's will, while 

 the others, by which the creature is enabled to move backwards 

 or forwards, depend entirely on its will. The rapidity of both 

 kinds of motion increases equally so long as the warmth is rising 

 up to 35°, but after that the backward or forward motion of 

 the animal ceases to be under the control of its will, and a 

 peculiar combination of aimless direct and rotatory motions is 

 the result. When the heat has risen to 40°', the direct motion 

 which became involuntary at 35° ceases entirel^^ while rotation 

 continues with undiminished vigour, till at last between 42° 

 and 45° heat-coma supervenes and death ensues. We see by 

 this that even two so closely allied functions as contractility 

 and ciliary motion, or even two very slightly different kinds of 

 cilia — those that work voluntarily and involuntarily — are in- 

 fluenced in widely different ways by the same increase of tem- 

 perature, exactly as individual animals react differently under 

 a rising temperature. Hence an increase of temperature, in 

 any district, resulting from secular variation, or an alt(;ration 

 in the distribution of heat as to the seasons, must exert 

 widely different influences on animals previously living to- 

 gether ; and the view so frequently expressed in the statement 

 that animals may be classed as tropical, sub-tropical, temperate, 

 and so forth — as if all the animals living together in the same 

 place must be equally well adapted to the climate of it, and as 

 if they must all react in the same way under any variations in 

 temperature — is evidently devoid of any solid foundation. 



We will now examine a few particular instances, to illustrate 

 more exactly what has been said by some extreme examples. 



We know that the optimum of temperature is not the same 

 for all animals,'" so we shall not be surprised to see that cer- 

 tain species can live in a temperature which to others is 



