476 
POPULAR SCIENCE REVIEW. 
long points on tlie wings and tail, — we see a high, concentra- 
tion of all the characters of a bird, as I have enumerated 
them above. Compare now with these the Guinea-fowl, with 
its short hollow wings, and small, weak, excavated breast- 
bone ; the tame duck, with its soft feathers and short, powerless 
tail; or the penguin, or the ostrich, in which the wings are 
reduced to mere rudiments, and have absolutely ceased to be 
flying organs ; and you will see what the ornithologist means 
when he speaks of the comparative perfection of these birds, 
when he says that the humming-bird is the most perfect 
representative of birds, the penguin the least. It is not that 
the penguin is not most admirably fitted for its mode of life 
and its requirements ; — nay, it exhibits some adaptations of 
structure to function which surprise us by their wondrous 
fitness. As a creature it is perfect, but not as a bird ; there 
is a manifest departure from the characteristic structure 
and functions of a bird, and an approach to those of some 
other great model of animate existence, while it still 
remains essentially a bird. It is, perhaps, the least perfect of 
birds. 
It is the same in the subordinate divisions. The swallows, 
taken as a family, are very perfect representatives of their 
class in the points above-mentioned ; but there are degrees of 
perfection among them. The swifts possess the characters 
which mark the family in a higher degree than the 
swallows ; while of these latter, how much more completely, 
“ more perfectly,” a swallow, is the chimney-swallow than 
the sand-martin. 
Haturalists have been accustomed to use the technical term 
“type,” or its adjective “typical,” to express the greatest 
aggregation of characteristic features in any given group. 
Among birds, the perchers constitute the typical order; in 
this order the cone-billed birds take the typical place as a 
tribe ; the finches, again, are the typical family of these ; and 
what is the type of these pretty little birds, as I am not 
writing an essay on ornithology, I shall leave to my readers* 
acumen to determine. 
There must always be some latitude in the determination of 
what constitute the typical characters of any given group, 
and therefore of the types themselves. One naturalist may 
picture to himself a different standard of completeness from 
that which another of equal eminence would assume ; and 
hence no one ought to speak oracularly; he merely gives 
his judgment. The great group of tiny creatures of which 
I am about to treat is, according to my judgment, the typical 
order of the class Rotifera. They may be designated, in 
homely parlance. The Flexible Creepers ; systematically, — • 
