292 THE PHILOSOPHY OF BIOLOGY 



were also armed by a heavy dermal exo-skeleton. It is 

 a hypothesis of considerable plausibility that they 

 really were Arthropods, on the other hand they are 

 usually regarded as Vertebrates. So also with most 

 other phyla : the morphological characters which 

 absolutely distinguish between one group and others 

 are very few indeed, and the small appended groups 

 that lie about the bases of these larger groups may 

 present one or other of the characters of several phyla. 

 Looking at the morphology of the animal kingdom in 

 a general kind of way, one does indeed see that a certain 

 structural plan is characteristic of the organisms be- 

 longing to each of the great phyla, while more detailed 

 structural plans may be said to be characteristic of the 

 sub-groups. But minute morphological and embryo- 

 logical investigation reduces almost to nothing the 

 characters which are absolutely diagnostic of these 

 various groups. 



No more than the nature of the energy-transforma- 

 tions, and the essential morphology, does the behaviour 

 of animals afford us the means of setting up absolute 

 distinctions between group and group. Really tropistic 

 behaviour is exhibited by the movements of the stems, 

 roots, and leaves of green plants, or in the movements 

 of Bacteria, and perhaps some unicellular animals. 

 Typically instinctive behaviour is exhibited by the 

 individuals of societies of Insects and by many solitary- 

 living animals belonging to this class ; and typically 

 intelligent behaviour is exhibited by the acting of the 

 higher Mammalia. Yet there is undoubtedly much 

 that is truly instinctive in the behaviour of Man, and 

 something of the same nature as his intelligence seems 

 to inhere in the instinctively- acting mammal or insect : 

 how else could an instinctive action become capable of 

 improvement ? We cannot doubt that intelligence is 



