Animal Causes. 231 



with the complexity of the animal's mechanism, and 

 especially with the complexity of its nervous and 

 sensory systems. 



We have seen that the sense organs and mental 

 machinery of the higher animals have been evolved 

 by their gastrula ancestors out of their epiblast, or 

 external integument or skin cells. It thus follows 

 that the epidermis of a low protozoon is a comp:e- 

 hensive sense-organ, combining in a primitive fashion 

 all the sense organs of the higher animals. This leads 

 us to ask, Is there a fundamental sense, a sense com- 

 prising all the senses, and of which all the more 

 specialised senses of the higher animals are but 

 offshoots ? 



The sense of touch may be so regarded. It is per- 

 ceived by us as well as the protozoon all over our 

 body, internally and externally. But higher animals 

 have, in addition to the gross sense of touch, subtler 

 organs of touch which we call sight, hearing, smelling, 

 and tasting. Yet when we analyse these senses, 

 their organs and their mechanism, we find that, funda- 

 mentally, the action of all the senses may be resolved 

 into touch and its re-action. The touches or vibrations 

 on the eye or ear are indeed surpassingly subtle, but 

 they are none the less touches. We are thus bound 

 to deduce that not only may the lowest or most 

 incipient form of animal intelligence be resolved into 

 touch and its re-actions, but we are bound to assume 

 also that any animal, even the highest, possessed as 

 all are of only tactile senses, can have no higher form 



