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POPULAR SCIENCE REVIEW. 
lyte , Alpheus , &c., so are species, genera, and families in general 
usually distinguished from their relatives and ancestors by 
numerous characters and not by a single one. And just as we 
can generally recognize no causal connexion between these 
various characters, so also in the case of Atyoida we should 
scarcely suspect anything of the kind, — for example, between 
the structure of the chelae, that of the posterior maxillae and 
the different armature of the carapace in the two sexes, had we 
only anatomized the dead animals. But so soon as we observe 
these little prawns living in all their activity, their mode of 
feeding at once explains the structure of their chelae and 
buccal organs and their residence on plants, and this, again, 
the many-clawed fingers of their walking feet ; we see how, as 
the structure of the chelae renders them unsuitable for the 
cleansing of the branchial cavities, the form of the posterior 
maxillae, which become adapted to this office, and the altered 
position of the first pair of swimming feet, come into connexion; 
the unarmed condition of the males enables us to understand 
their smaller size, and this again, the youthful structure of 
their carapace, &c. &c. ; in short, all their manifold peculiarities 
at once come into the closest relation to one another, and we 
may hope that in many other cases, also, by careful observations 
of living animals, many apparently unconnected structures, for 
which at present a mysterious correlation of parts is made 
responsible, will come to be recognized as reciprocally causative 
results of Natural Selection. 
