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 both 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 aerial 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 Mollusc and a Fish can scarcely be compared ; 

 still less can those of a Tunicate and a Reptile, or of an Infusory 



