420 THE PRINCIPLES OF SCIENCE. 



groundwork of similarity running through all classes, 

 but particular actions and processes present themselves 

 conspicuously in particular families and classes. Tenacity 

 of life is most marked in the Rotifera, and some other 

 kinds of microscopic organisms, which can be dried and 

 boiled without loss of life. Eeptiles are distinguished 

 by torpidity, and the length of time they can live without 

 food. Birds, on the contrary, exhibit ceaseless activity and 

 high muscular power. The ant is as conspicuous for 

 intelligence and size of brain among insects as the quad- 

 rumana and man among vertebrata. Among plants the 

 Leguminosee are distinguished by a tendency to sleep, 

 folding their leaves at the approach of night. In the 

 genus Mimosa, especially the Mimosa pudica, commonly 

 called the sensitive plant, the same tendency is magnified 

 into an extreme irritability, almost resembling voluntary 

 motion. More or less of the same irritability probably 

 belongs to vegetable forms of every kind, but it is of 

 course to be investigated with special ease in such an 

 extreme case. In the Gymnotus and Torpedo, we find that 

 organic structures can act like galvanic batteries. Are we 

 to suppose that such animals are entirely anomalous ex- 

 ceptions ; or may we not justly expect to find less intense 

 manifestations of electric action in all animals and 

 plants 1 



In the animal world we find many phenomena which 

 seem to be peculiar to certain classes, but are afterwards 

 found to differ but in degree from what is always present. 

 The lower animals, for instance, seem to differ entirely 

 from the higher ones in the power of reproducing lost 

 limbs. A kind of crab has the habit of casting portions of 

 its claws when much frightened, but they soon grow again. 

 There are multitudes of smaller animals which, like the 

 Hydra, may be cut in two and yet live and develop into 

 new complete individuals. No mammalian animal can repro- 



