56' MENTAL EVOLUTION IN ANIMALS. 



advance in the mechanism of adaptive movement. Number- 

 less other cases of such simple powers among plants might 

 here be noticed ; but none of them rise above the level of 

 distinguishing between one or two alternatives of stimula- 

 tion, and supplying the correspondingly simple movements 

 of response. Where nerve-structure first appears, we find 

 that the animals which present it — the Medusse — have organs 

 of special sense wherewith to distinguish with comparative 

 delicacy and rapidity between light and darkness, and 

 probably also between sound and silence. They are also 

 provided with an elaborate tentacular apparatus, wherewith 

 they are able to distinguish quicldy and accurately between 

 moving and not moving objects coming upon them from 

 various sides, as well as between nutritious and non-nutri- 

 tious particles. And in correspondence with this advance of 

 receptive capacity we observe a considerable advance of 

 executive capacity — the animals being highly locomotive, 

 swinmiing away rapidly from sources of contact which they 

 distinguish as dangerous, and manifesting several other reflex 

 actions of a similarly adaptive kind. Thus, also, the higher 

 organizations of Star-fish, Worms, &c., while serving to supply 

 the neuro-muscular mechanisms with still more detailed 

 information regarding the outer world, serve likewise to 

 supply them with the means of executing a greater variety 

 of adaptive movements. In the Mollusca, again, we observe 

 another advance in both these respects; the animals feel 

 their way with sensitive feelers, select varied kinds of food, 

 choose mates of their own species to pair with, and may even 

 remember a particular locus as their home, &c. Among the 

 Articulata the lower forms present co-ordinated movements 

 wdiich are few and simple as compared with the many and 

 varied movements of the higher members of the class ; and 

 their powers of distinguishing between stimuli are propor- 

 tionally small. But in the complicated anatomy of the 

 Crabs and Lobsters there is a large provision for the co-ordina- 

 tion of movements, and the selective actions are correspond- 

 ingly numerous and varied; while among the Insects and 

 Spiders the power of muscular co-ordination surpasses that 

 of the lower Vertebrata, and the power of intelligent adapta- 

 tion, assisted by delicate antennae and highly perfected organs 

 of special sense, is also greater. And the same principles 

 hold throughout the Yertebrated series. It has already been 



