730 THE PRINCIPLES OF SCIENCE. [chap. 



ducing lost limbs. A kind of crab 1ms 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 reproduce a limb, and in appearance there is no 

 analogy. But it was suggested by Blumenbach that the 

 healing of a wound in the higher animals really represents 

 in a lower degree the power of reproducing a limb. That 

 this is true may be shown by adducing a multitude of in- 

 termediate cases, each adjoining pair of whicli are clearly 

 analogous, so that we pass gradually from one extreme to 

 the other. Darwin holds, moreover, that any such re- 

 storation of parts is closely connected with that perpetual 

 replacement of the particles which causes every organised 

 body to be after a time entirely new as regards its con- 

 stituent substance. In short, we approach to a great 

 generalisation under wliich all tlie phenomena of growth, 

 restoration, and maintenance of organs are effects of one 

 and the same power.^ It is perhaps still more sur- 

 prising to find that the complicated process of reproduc- 

 tion in the higher animals may be gradually traced down 

 to a simpler and simpler form, which at last becomes un- 

 distinguishable from the budding out of one plant from the 

 stem of another. By a great generalisation we may regard 

 all the modes of reprnduction of organic life as alike iu their 

 nature, and varying only in complexity of development.^ 



Limits of Classification. 



Science can extend only so far as the power of accurate 

 classification extends. If we cannot detect resemblances, 

 and assign their exact diameter and amount, we cannot 

 have that generalised knowledge which constitutes science ; 

 we cannot infer from case to case. Classification is the 

 opposite process to discrimination. If we feel that two 

 tastes differ, the tastes of two kinds of wine for instance, 

 the mere iact of difference existing prevents inference. 

 The detection of the difference saves us, indeed, from false 



' Darwin, The Variation of Animals and Plants, vol. ii. pp. 293, 

 359, kc. ; quoting Paget, Lectures on Pathology, 1853, j^p. 152, 164. 

 ^ Jbid. vol. ii. p. 372. 



