CHAP. X. ALASMODONTIN^:. 291 



the dark ages of zoology, and produce the same sort of 

 confusion and ignorance which then reigned. But, then., 

 it may he asked, how can genera be retained in our 

 systems, which actually blend into each other, and 

 whose distinctions thus become lost ? This objection 

 has been repeatedly answered. Genera which are really 

 natural, are always thus united, because they show us 

 all the connecting links of nature. But genera which 

 contain only well-defined and isolated characters, so 

 that no ordinary person can confound them, are not 

 perfect, because their connecting links to others are 

 wanting : they are fragments of the chain, having its 

 gradations disturbed, dissevered, broken ; hence such 

 groups are fragments only of what has been, or may be, 

 a natural assemblage. Our divisions of sub-families, 

 genera, sub-genera, and sections, are mere conventional 

 terms employed to denote groups of different sizes, pos- 

 sessing, in the majority, certain primary characters which 

 are termed typical. It matters not upon what cha- 

 racters these divisions are founded in the Unionidce 

 and the Helicida, provided none can be drawn from 

 the animal ; but it is most essential that disruptions of 

 kindred species should not be made, for then the 

 harmony of nature is destroyed. Now, this leads us 

 again to advert to the different sorts of resemblances 

 natural objects bear to each other. The confounding 

 of analogy and affinity, which has led to the greatest 

 confusion in every branch of zoology, has been particu- 

 larly conspicuous in this : every conchologist, for in- 

 stance, perceives that Ligumia recta, Mysca ovata and 

 nasuta, Iridina ekmgata, and Alasmodon margaritifera, 

 have a very strong resemblance to each other. Now, if 

 this resemblance were one of affinity, it naturally would 

 follow that all these shells belonged to the same natural 

 group : they possess one character in common, that of 

 being particularly long, pod-like shells ; yet if any me- 

 thodist, looking to this only, and disregarding all other 

 characters, make them into a genus, he would commit 

 a palpable outrage on natural classification. So, in like 

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