124 CHAPTERS ON EVOLUTION. 



instance. But the conditions in virtue of which a quadruped like the 

 bat has acquired its powers of flight may have been, and probably were, 

 different in nature, as they certainly were in time, from those under 

 which the bird learned to soar in the air. This latter point, however, 

 is foreign to the main issue before us. Sufficient for our present 

 purpose are the thoughts, that homology and analogy are two 

 distinct things ; that homology indicates the deeper and real like- 

 ness between organs and parts ; and that these two forms of likeness 

 are not necessarily connected or coexistent. 



So much by way of introduction to the subject of the science of 

 likenesses. It requires but little guidance to enable the mind to 

 follow up the line of thought already mentioned in the preceding 

 remarks, which shows the function of this branch of inquiry in 

 detecting the hidden relationships and bonds which connect one 

 living being with another, or one class of organisms with a neighbour- 

 ing class. Such relationships, as every one knows, are indicated by 

 the systems of classification and arrangement which form an im- 

 portant part of every science, and, one may add, of many matters 

 connected with every-day existence as well. Thus, the classification 

 of the objects under his study or care is equally important for 

 botanist and librarian ; and in either case the aim of the system 

 of arrangement is to bring together things that are like, and to sepa- 

 rate those that are unlike. It matters not how this procedure is 

 effected. Classifications vary with well-nigh each person who under ^ 

 takes their formation ; and the needless multiplication of systems of 

 arrangement, equally with the persistent invention of new cognomens 

 for already well-named species, constitute the two chief sorrows of 

 the well-regulated scientific mind. The best classification is of 

 course the " natural; " but it so happens that this particular arrange- 

 ment is not always easy of construction : a fact chiefly explicable on 

 the ground that the natural relationships of living beings are often 

 hard to seek and difficult to find. When the popular classification 

 of the fish with the whale one, it may be added, not characteristic 

 of primitive minds alone is replaced by the union of the whale 

 with the quadrupeds seeing that it has warm blood, brings forth its 

 young alive, and nourishes them by means of milk a grossly artifi- 

 cial system of arrangement is superseded by a true and natural one. 

 That a whale need not be a fish because it swims, or is fish-like, is thus 

 evident; and the correctness of our arrangement of whales and fishes, 

 and of the whole animal and plant worlds, must of necessity depend on 

 the completeness of our knowledge of the objects we intend to classify. 



Now, it is exactly the difficulties which stand in the way of form- 

 ing a natural arrangement of animals and plants which are lightened 

 by the study of homology as the science of likenesses. From 

 the mere arrangement and classification of living beings, it may 



