PROFESSOR VIRCHOW AND EVOLUTION. 379 



tudinal vibration. Now the substitution of transverse 

 for longitudinal vibrations in the case of light involved 

 a radical change of conception as to the mechanical 

 properties of the luminiferous medium. But though 

 this change went so far as to fill space with a substance, 

 possessing the properties of a solid, rather than those of 

 a gas, the change was accepted, because the newly dis- 

 covered facts imperatively demanded it. Following Mr. 

 Martineau's example, the opponent of the undulatory 

 theory might effectually twit the holder of it on his 

 change of front. ' This ether of yours/ he might say, 

 * alters its style with every change of service. Starting 

 as a beggar, with scarce a rag of ' property ' to cover its 

 bones, it turns up as a prince when large undertakings 

 are wanted. You had some show of reason when, with 

 the case of sound before you, you assumed your ether to 

 be a gas in the last extremity of attenuation. But now 

 that new service is rendered necessary by new facts, you 

 drop the beggar's rags, and accomplish an undertaking, 

 great and princely enough in all conscience; for it im- 

 plies that not only planets of enormous weight, but 

 comets with hardly any weight at all, fly through your 

 hypothetical solid without perceptible loss of motion/ 

 This would sound very cogent, but it would be very vain. 

 Equally vain, in my opinion, is Mr. Martineau's conten- 

 tion that we are not justified in modifying, in accord- 

 ance with advancing knowledge, our notions of matter. 

 Before parting from Professor Knight, let me com- 

 mend his courage as well as his insight. We have 

 heard much of late of the peril to morality involved in 

 the decay of religious belief. What Mr. Knight says 

 under this head is worthy of all respect and attention. 

 'I admit/ he writes, 'that were it proved that the 

 moral faculty was derived as well as developed, its 

 present decisions would not be invalidated. The child 

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