98 MENTAL EVOLUTION IN ANIMALS. 



in those parts to be more sensitive to changes of temperature. 

 iBut the deposition of pigment in such localities constitutes a 

 ;favouring condition to the origination of an eye, or of an 

 ,'organ whose sense of temperature becomes sufficiently 

 'developed to enable it to begin to distinguish between light 

 and darkness. Thus, as Professor H?eckel eloquently re- 

 marks : " The ordinary nerves of the skin which pass to these 

 dark pigment-cells of the integument, have already trodden 

 the first steps of that magnificent march, at the end of wliich 

 they have attained to the highest development of the nerves 

 of sensation — the optic nerves." 



Turning next to the sense of Colour, it appears from the 



• experiments of Engelmann already alluded to, that colour- 

 j sense of a kind occurs as low down in the zoological scale as 

 • the protoplasmic and unicellular organisms, inasmuch as 

 particular species showed particular preferences for certain 

 rays of the spectrum. But as in these organisms there are 

 no organs of special sense, a ad probably no beginnings of 

 consciousness, I do not think that any true analogy can be 

 drawn between these cases and those in which there is a true 

 sensation of colour. Nor have we any evidence of such a 

 true sensation till we arrive at the Crustacea. Here we 

 have proof, furnished by the direct experiments of Sir John 

 Lubbock, that Daphnia pulex prefers certain rays of the 

 spectrum to others,* and the Chameleon Shrimp (Mysis cha- 

 meleo) is known to cliange its colour in imitation of the 

 surface on which it reposes, provided that it is not blinded 

 or otherwise prevented from seeing that surface. Precisely 



J^ analogous facts occur among the Cephalopoda {e.g., odojy^is), 

 Batrachia (e.g., Common Frog), Pteptilia (e.g., Cameleon), and 

 Pisces (e.g.. Flounder) ; in all these cases, if the animals are 

 blinded, the effects no longer occur. Moreover, Pouchet 

 found that in the Pleuronectidse the mechanism whereby 

 these imitative changes of colour are produced is bilaterally 

 disposed, so that if only one eye of the animal is stimulated 

 by coloured light, only one side of the animal changes colour. 

 M. Fredericq afterwards found the same thing to be true of 

 the Octopus, and in conjunction with Professors Burdon- 



* Joiirn. Linn. Soc, 1881. These observations have been adversely criti- 

 cized by Merejkowsky {Comj)tes Hendus, xciii, pp. 160-1), but his criticisms 

 have been fully met by further experiments recently pubhshed by Sir John 

 {Journ. Linn. Soc, 1883). 



