190 ESSAY ON CLASSIFICATION 



winged; and many Fishes which are capable of rising above the 

 water are also described as winged. But the wing of a bat is homolo- 

 gous to the foreleg of the other Mammalia; and only analogous to 

 that of Birds; for it exhibits the special homologies of the class of 

 Mammalia, and not those of the class of Birds.^^ The same is true of 

 the so-called wings of the flying fishes, in which the wing is a fin, 

 homologous to the pectoral fin of other bony fishes, and not con- 

 structed in the same way as the wing of the bat or that of the bird. 

 The wing of Insects is entirely different, and its analogy with the 

 wing of birds more remote than that of the bat and of the flying 

 fish, inasmuch as it is not an analogy between members of different 

 classes of the same branch, but between two classes of different 

 branches, differing therefrom in the plan of structure, and not only 

 in the mode of execution of one and the same plan. 



Ordinal analogies. As orders are founded upon the complications 

 of the structure which characterizes the different classes, it is not 

 likely that ordinal analogies will occur between the different orders 

 of one and the same class; we may rather expect them to be promi- 

 nent between the orders of closely allied classes or between the orders 

 of a higher class and the lower classes of the same branch. We find, 

 for instance, a remarkable correspondence between the orders of the 

 class of Batrachians, and those of the class of true Reptiles.^^ The 

 same may be said of the order of Cetacea in the class of Mammalia, 

 as compared to the whole class of Fishes, or of the lower order of the 

 Insects (the Myriapods) as compared to the class of Worms, or of the 

 lower order of Acalephs (the Hydroids) as compared to the class of 

 Polyps.3^ An accurate knowledge of these kinds of analogies is of the 

 utmost importance for the study of the true affinities of animals, 

 since a misapprehension of the real value of their structural features 

 has again and again misled zoologists into combining such groups as 

 if they were truly related. In the beginning of the last century, for 

 instance, the Cetacea were generally united with the Fishes, to which 

 they are only analogous; and even to this day we see the Hydroids, 



'* As limbs of Vertebrates these two kinds of wings are, nevertheless, homologous; but 

 as wings they are only analogous. 



^ For further details see the second Part of the first volume of my Contributions 

 (1857), Sect. Ill, pp. 252-255. 



^"For further details I must refer to the third volume of my Contributions now in 

 the press [Boston, I860.] 



