80 MENTAL EVOLUTION IN ANIMALS. 



sense, I shall next proceed to take a brief survey of the 

 animal kingdom with reference to the powers of special 

 sense. In doing this, however, it is needless, and indeed 

 undesirable, that I should enter with much closeness into the 

 anatomy of the innumerable organs of special sensation 

 which the animal kingdom presents. My object is merely to 

 give a general outline of the powers of special sensation pro- 

 bably enjoyed by different classes of animals ; for, as these 

 powers constitute the foundation of all the other powers of 

 mind, it is of importance for us to have a general idea of the 

 grade of their development in the sundry grades of the 

 zoological scale. 



In some of his recently published experiments, Engel- 

 mann found that many of the protoplasmic and unicellular 

 organisms are affected by light ; that is to say, their move- 

 ments are influenced by light, in some cases causing accele- 

 ration, in others slowing, of their movements ; in some cases 

 the organisms seeking the light, while in other cases they 

 shun it, &c, &c. He found that all these effects were re- 

 ducible to one or other of three causes : (1) alteration pro- 

 duced by the light in the interchange of gases, (2) consequent 

 alteration in the conditions of respiration, and (3) specific 

 processes of luminous stimulation. It is with the latter only 

 that we are concerned, and the organism which Engelmann 

 names as exhibiting it typically is Englena viridis. After 

 precautions had been taken to eliminate causes 1 and 2, it 

 was still found that this organism sought the light. More- 

 over, it was found that it would only do so if the light were 

 allowed to fall upon the anterior part of its body. Here 

 there is a pigment-spot, but careful experiment showed that 

 this was not the point most sensitive to light, a colourless 

 and transparent area of protoplasm lying in front of it being 

 found to be so. Hence it is doubtful whether this pigment- 

 spot is or is not to be regarded as an exceedingly primitive 

 organ of special sense. Of the rays of the spectrum, Englena 

 viridis prefers the blue.* 



The remarkable observation recorded by Mr. H. J. Carter, 

 F.R.S., and quoted from him in my previous work,f seems to 

 display almost incredible powers of special sense among the 



* For full account of these experiments, see rjtiiger's Archiv.f. d.ges. 

 Physiologie, Bd. XXIX, 1882. 



f Animal Intelligence, pp. 19-21. 



