88 Dr. C. Collingwood on Recurrent Animal Form, 
bird ; but its wing, not being formed upon the type of that which 
exists in a true bird, must be inferior ; nevertheless it is as truly 
and completely a wing as is the far more perfect, but less bulky, 
wing of a bird. 
Further, if we select a single Class, such as the Mammalia, and 
bear in mind the same principle, we shall find it lead to the same 
results. Some quadrupeds of each Order are arboreal, some 
terrestrial, and others subterranean ; some are carnivorous, some 
insectivorous, and some frugivorous ; some are nocturnal, some 
diurnal, and some crepuscular. If, now, an animal belonging to 
one Order is, like an animal of a different Order, insectivorous, 
the former probably bears some remote analogy to the latter, by 
virtue of that fact. If the animals of two different Orders are 
not only doth insectivorous, but also crepuscular, for example, 
the probability of their resemblance is increased ; but if the two 
are insectivorous, crepuscular, and subterranean, then the great 
agreement of their habits must be accompanied by a considerable 
approximation of form. 
Perhaps there are no facts in the natural history of animals 
which are simpler, or with which we are more familiarly ac- 
quainted in a general way, than the broad characteristics which 
differentiate the habits and modes of life of quadrupeds, birds, 
and fishes ; and, on the other hand, the aberrant forms which are 
assumed by aquatic mammals and birds, and by aérial quadru- 
peds, and the homomorphism of these aberrant forms with those 
of the classes of Vertebrata which they most nearly approach in 
their habits and modes of life, are highly important questions, 
which thus admit of elucidation with a degree of probability 
commensurate with this exactness of our knowledge of those 
habits. The kind of homomorphism which obtains between 
members of a Class, such as among the various Orders of the 
Mammalia, requires a different kind of knowledge, viz. not a 
general aquaintance with broad facts, but a special familiarity 
with individual habits. Now, such a special knowledge is by no 
means always possessed, or even easily attainable ; but when it is 
so, it is found that the greater the agreement of habit and modes 
of life between any two animals of distinct Orders, the more 
striking is the homomorphism which exists between them. Of 
this proposition several illustrations have already been given. 
Taking now our stand upon these .facts, and carrying the 
principle which I have laid down into the Invertebrate division 
of animals, the first thing which strikes us is the comparative 
artificiality of some of the resemblances which might be instanced 
as existing between them and the Vertebrate subkingdom.. 
The habits of a Molluse and a Fish can scarcely be compared ; 
still less can those of a Tunicate and a Reptile, or of an Infusory 
eae 
