32 NATURAL SCIENCE. July, 



It is noteworthy that all these were either females or immature 

 males ; the adult males are normally very large, usually at least twice 

 the size of females, and are provided with long and powerful chelae, 

 which make them independent of the subterfuges adopted by the weak 

 small females. An interesting point, to J, is that the weak chelae of these 

 mimics are quite smooth, without the recurved bristles of the other 

 limbs, and hence are unlike them, never having any weedy trimmings. 



II. — Colour Assimilation Among Fishes. 



We already know a good deal about this, especially among the 

 flat-fishes, so the following instances but go to swell an already long 

 list. Still, these are so striking as to be worthy of permanent record. 



Two tanks were used for experiment, one with dark background 

 and bottom, well shadowed ; the other bright, with a white mottled 

 sand bottom. Several of the marine stickleback (Gasterosteus spinachia) 

 were placed in each. To sum up the result briefly, those in the dark 

 shadowed tank remained practically unchanged in colour, but those 

 in the light-coloured tank had in greater or less degree lost their 

 brightness and intensity of colouring. The beautiful gold bronze 

 lustre so characteristic of these sticklebacks was lost, and the backs 

 were mottled black and white, contrasting strangely with the nearly 

 unbroken yellowish black of the dorsal surface of their friends in the 

 dark tank. 



In the dark tank had also been placed a number of Wrasses 

 (Labridae), and these showed fading all round, most marked in the 

 bright greens and scarlets. As these colours are usually in com- 

 bination with brownish marking, the fading of the bright hues meant 

 a close approximation to the brown appearance of the bare con- 

 glomerate forming the rockwork of the tank. One fish especially 

 beautiful at first — of a most brilliant scarlet and brown — faded to a 

 dirtv combination of pale olive green and brown, scarcely recognisable 

 had the fish not been marked in a distinctive manner at the beginning 

 of the experiment. The whole of these colour changes were effected 

 within the remarkably short period of a week. 



It may be that these instances of colour assimilation carry the 

 key to the problem of colour variation or rather mimicry in the prawn 

 Hippolyte (Virbius) varians. 



Plaice (Pleuronectes platessa) have also shown rapidity of colour- 

 change much more marked than I was prepared for. Some that were 

 placed in a large shallow tidal pond where the colour of the bottom 

 varies considerably and where a portion is often in deep shadow, 

 show change from a uniform grey to a well-marked and intensely 

 dark blotched appearance within a few seconds. Indeed, it is quite 

 chameleon-like, so quickly is the transformation effected. Inordinary 

 tanks where the light and the colour of the sand are stable, the plaice 

 soon take the exact colouring requisite, and retain it without altera- 

 tion so long as they remain in the particular tank. 



