192 SENSATION, IDEATION, 



heritance ' of any particular nervous structures carries with 

 it the possibility of manifesting (on fit occasions) the func- 

 tional activities with which such structures have previously 

 been in relation in ancestral forms — whether these activi- 

 ties have been concerned with more or less complex Mental 

 Phenomena, or in evoking complex Muscular Movements. 

 « We have seen that the Nervous Systems in Invertebrates 

 generally, are composed of Ganglia variously connected 

 together, some of which are in relation, by means of 

 * ingoing ' nerves, with impressible surfaces or special 

 Sense Organs ; while others are in relation with Muscles 

 and Glands through the intervention of ' outgoing ' nerve 

 fibres. Such surfaces and organs are, therefore, the inlets 

 of all imj)ressions by which the Organism attains a Know- 

 ledge of, or, in other words, is brought into relation with, 

 the External World. The celebrated phrase of the school- 

 men, 'nihil est in intellectu quod nonfiierit j^riusin sensu,^ 

 w^ould seem, therefore, — so far as it is applicable at all — 

 to be a mere truism for all lower animals, whose Brain is 

 represented by a mere congeries of Ganglia in connection 

 with impressible surfaces and more special Sense Organs. 



But we have seen how these Sensory Ganglia increase 

 in size and tend to grow into closer relations with one 

 another in higher Mollusks and in Insects ; how in Fishes, 

 Beptiles, and Birds they undergo a still further develop- 

 ment in each class — nay, more, how in these lower Verte- 

 brates certain new co-ordinating ganglia known as Cere- 

 bral Lobes appear, in intimate structural relation with 

 the several sensory ganglia, and which notably increase in 

 their proportional size as we pass from Fishes to Eeptiles, 

 and from Ptcptilcs to Birds. 



But let the reader also bear in mind v;hat has been set 

 forth in the present chapter- — How philosophers in all 

 times have recognized that no Impressions or Sensaticns 



