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night to recommend. Let us turn away then from 

 abstract definitions. Let us take some concrete living 

 thing, some animal, the commoner the better, and let us 

 see how the application of common sense and common 

 logic to the obvious facts it presents, inevitably leads us 

 into all these branches of zoological science. 



I have before me a lobster. When I examine it, what 

 appears to be the most striking character it presents? 

 Why, I observe that this part which we call the tail of 

 the lobster, is made up of six distinct hard rings and a 

 seventh terminal piece. If I separate one of the middle 

 rings, say the third, I find it carries upon its under sur- 

 face a pair of limbs or appendages, each of which con- 

 sists of a stalk and two terminal pieces. So that I can 

 represent a transverse section of the ring and its appen- 

 dages upon the diagram board in this way. 



If I now take the fourth ring I find it has the same 

 structure, and so have the fifth and the second ; so that, 

 in each of these divisions of the tail, I find parts which 

 correspond with one another, a ring and two appendages ; 

 and in each appendage a stalk and two end pieces. 

 These corresponding parts are called, in the technical 

 language of anatomy, " homologous parts." The ring 

 of the third division is the " hornologue " of the ring 

 of the fifth, the appendage of the former is the homo- 

 logue of the appendage of the latter. And, as each 

 division exhibits corresponding parts in corresponding 

 places, we say that all the divisions are constructed upon 

 the same plan. But now let us consider the sixth di- 

 vision. It is similar to, and yet different from, the 

 others. The ring is essentially the same as in the other 

 divisions ; but the appendages look at first as if they 

 were very different ; and yet when we regard them 

 closely, what do we find ? A stalk and two terminal 

 divisions, exactly as in the others, but the stalk is very 



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