PROTECTIVE COLOHATIOK. 127 



have been produced through natural selection. It is supposed 

 that variations in the direction of a more perfect resemblance 

 have survived, and that by a cumulative effect, which may 

 have taken generations to produce, the wonderfully faithful 

 likenesses between many caterpillars and the twigs of their 

 food-plants, etc., have been arrived at. 



There are, however, certain cases to which this line of 

 reasoning cannot well apply. 



Dr. Eisig* found an annelid of the genus Eunice living: 

 parasitically upon a marine sponge in the Bay of Naples. 

 The sponge is of a yellow colour, caused by the presence in its 

 tissues of small particles of a colouring-matter in all proba- 

 bility belonging to that class which I have referred to in 

 Chapter I. as of physiological importance. The annelid is 

 similarly coloured, its body being decked with numerous 

 orange spots. This resemblance is not comparable to that 

 between Acolosoma and various filamentous plants ; for the 

 pigment is identical, and has been simply transferred from the 

 tissues of the sponge to the skin of the worm, having iirst 

 traversed a portion of the alimentary canal of the latter. 



It is possible that this explanation of protective resem- 

 blances may be applicable to many other cases, and do away 

 with the necessity of assuming any special action of natirral 

 selection. 



In an interesting series of articles upon protective colours in 

 the Nudibranchiate Mollusca,t Prof. Herdman has described in 

 some detail the habits of these creatures. Many nudibranchs 

 are furnished with simple or branched processes upon the 

 back ; these forms are chiefly carnivorous, feeding upon hydroid 

 polyps, among the colonies of which they may be frequently 



* " Fauna und Flora des Golfes von Neapel : Die Capitelliden.' 

 t Life-Lore for 1890. 



