52 SPECIES NOT 



And in the next passage: "I can hardly indeed 

 doubt that all vertebrate animals having true 

 lungs, [at the head of which is man,] have des- 

 cended by ordinary generation from an ancient 

 prototype, of which we know nothing, furnished 

 with a floating apparatus, or swim bladder." An 

 organ now at all events exclusively confined to 

 fishes, animals living in water. 



But to prevent any doubt of Mr. Darwin's 

 views on this subject, I will quote one more 

 passage. Alluding to the doctrine u Xatura non 

 facit saltum," he remarks: "Why on the the- 

 ory of creation should this be so? Why should 

 all the parts and organs of many independent 

 beings, each supposed to have been separately 

 created for its proper place in nature, be so in- 

 variably linked together by graduated steps? 

 Why should not nature have taken a leap from 

 structure to structure? On the theory of natural 

 selection we can clearly understand why she 

 should not, for natural selection can act only 

 by taking advantage of slight successive varia- 

 tion; she can never take a leap, but must ad- 

 vance by the shortest and slowest steps." 



The ground, then, is cleared of all doubt as 

 to Mr. Darwin's meaning. The fish has been 

 converted into the lung-breathing animal. But 

 we have lost the prototype, says Mr. Darwin. 

 How is this? He argues from an existing type, 

 that certain changes have taken place between 

 the swim bladder and the lung, ending in the 

 conversion of the one into the other. But why 



