RETROGRESSION. 173 



likenesses between the tiger and the cat, which affect all 

 the more important features of the anatomical structure 

 of these two animals. 



Though Swainson recognised in theory the distinction 

 between these two kinds of likenesses now known 

 respectively as likenesses of * analogy ' and likenesses of 

 * homology ' he showed, by his loose application of these 

 in practice, that he did not sufficiently recognise the causes 

 of the distinction between them. As modern naturalists 

 understand this matter, likenesses of ' analogy ' are purely 

 physiological or adaptive, and depend merely upon similarity 

 of mode of life or external environment. Thus, whales are 

 like fishes because both have certain structural modifica- 

 tions, as regards the form of the body and shape of the 

 limbs, which adapt them for a life in a watery medium. 

 Similarly, there is a certain resemblance between bats and 

 birds, due to the fact that both are adapted for flight in 

 the air. These physiological or analogical likenesses, 

 however, are quite independent of real relationships, and 

 they are therefore of no value for classificatory purposes. 

 On the other hand, there are likenesses between animals 

 which are morphological, and which are quite independent 

 of the kind of life which the animal may lead, or the 

 nature of its surroundings. These ' homological ' like- 

 nesses are dependent upon identity of structure and funda- 

 mental plan, and they exist irrespective of, and despite of, 

 the animal's habit of life or the particular use to which it 

 may put its organs. Thus, to give a single example, there 

 exists a homological likeness, due to identity in funda- 

 mental plan of construction, between a butterfly and a 

 lobster; though these animals are by no means like one 



