480 PHILOSOPHIC ZOOLOGY 



of it is that such a tree is really a genealogical one. The con- 

 clusion is fully confirmed by the geological record. 



We also find a number of existing animals which, though on 

 the whole susceptible of classification in one group, also show 

 points of marked agreement with members of one or more other 

 groups. A notable instance is afforded by Peripatus (see vol. i, 

 p. 398), which, though an undoubted Arthropod, is singularly 

 like a segmented Worm or Annelid in some respects. If the 

 classification tree is a genealogical one, the existence of such 

 animals is readily intelligible. Such cases are otherwise inex- 

 plicable, unless some unintelligible and dogmatic statement offered 

 by the believer in special creation can be so regarded. 



THE ARGUMENT FROM FORM AND STRUCTURE (MORPHOLOGY). 

 A very large number of examples might be brought forward 

 to show that many organs can only be rationally interpreted on 

 an evolutionary basis. A particularly good instance is afforded 

 by the lungs of air-breathing vertebrates, which appear to be 

 modifications of the swim -bladders possessed by fishes (see 

 vol. ii, p. 421). And it may be added that there are many 

 other structural characters of these air-breathing forms which 

 point to an aquatic ancestry. That this should be so, is only 

 intelligible from the stand-point of evolution. 



If we take a particular group of animals, say Mammals, we 

 shall find that they are constructed on a particular plan, modified 

 in a great variety of ways to suit the exigencies of various modes 

 of life. This is very well illustrated by the structure of the limbs, 

 in reference to different kinds of locomotion, e.g. swift progression 

 by running on a firm surface, swimming, climbing, and burrow- 

 ing, as set out in detail in the section on Locomotion (vol. iii). 

 We have here, it would appear, a gradual Adaptation by a pro- 

 cess of evolution to conditions of different kind. 



The strongest argument from structure in favour of the 

 doctrine of evolution is that derived from those parts of animals 

 which are known as vestiges (rudimentary organs). The human 

 body, for example, is in itself quite a museum of such structures. 

 Indeed, one may say that it is an archaeological museum, for 

 vestiges can only be reasonably explained as the remains of 

 organs which were of greater importance in ancestral forms. 

 The lower end of the backbone (coccyx), for instance, looks 

 uncommonly like the remains of what was once a tail, and the 



