50 CHAPTERS ON EVOLUTION. 



these organs are so modified as to perform a much greater variety of 

 functions, while they are all co-ordinated towards the maintenance of 

 the animal by its well-developed nervous system and sense organs. 

 But," concludes Huxley, "it is impossible to say that, e.g., the 

 Arthropoda (insects, spiders, centipedes, &c.) as a whole are 

 physiologically higher than the Mollusca (shell-fish and cuttle-fishes), 

 inasmuch as the simplest embodiments of the common plan of the 

 Arthropoda are less differentiated physiologically than the great 

 majority of Mollusks." 



Whilst the difficulties which lie in the way of determining the 

 higher or lower rank of many organisms are thus apparent, it may 

 be remarked that the means already specified namely, the 

 physiological perfection of the animal may in turn assist us in 

 assigning to many forms their relative place in any type or group. 

 Thus the possession of air-breathing organs, or lungs, is admit- 

 tedly a mark of a higher organisation than that which possesses 

 gills j the life of the air-breather being, as a matter of fact, associated 

 with a structural perfection excelling that of the aquatic and gill- 

 bearing animal. So also the degradation of organs and parts which 

 accompanies parasitism, naturally lowers an animal in the series as 

 compared with its non-parasitic neighbours. Thus within the 

 limits of any one type of animals, we may discover many examples of 

 tendencies to higher as well as to lower development, these tend- 

 encies determining the position of the organism within its own type, 

 and either elevating itself or its group collectively, or, on the other 

 hand, degrading it, and assigning it to a low place in the type. The 

 impossibility of any scientific or natural arrangement of animals in a 

 linear series can thus be shown to depend simply upon the con- 

 stitution of the animal kingdom as a whole. If any arrangement of 

 the great types it presents to view is permissible and naturalists are 

 agreed that some such relationship is embodied and included in the 

 constitution of the kingdom such an arrangement will find its 

 clearest expression in the metaphor of a tree. As represented, 

 indeed, in the foregoing table (page 47), the various types may be 

 regarded as the great branches of the animal tree, rising here and 

 there from a common stem or root, and developing, each along its 

 own special line and type, into the variety and fulness of form 

 exhibited before our eyes to-day. 



The impression which is liable to be left on the mind of the 

 observer who, thus far, has traced out the constitution of the animal 

 world into its fundamental types or plans, will undoubtedly take the 

 form of the idea that the mere existence of these types or plans as 

 we behold them represented in living animals would appear to 

 indicate the separate and disconnected nature of the great groups in 

 question. Considerations of this nature inevitably lead to others, 



