PROFESSOR VIRCHOW AND EVOLUTION. 379 



vibration. Now the substitution of transverse for longi- 

 tudinal vibrations in the case of light involved a radi- 

 cal 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 discovered 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 implies 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 contention that 

 we are not justified in modifying, in accordance with 

 advancing knowledge, our notions of matter. 



Before parting from Professor Knight, let me 

 commend 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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