NOTICES OF NEW BOOKS. 439 



best general account of these creatures which has yet been 

 written. We refer more particularly to this portion of the 

 publication because it can be read with interest and profit by 

 all zoologists, even if they possess no special knowledge them- 

 selves of the Nudibranchiate Mollusca. It embraces such sub- 

 jects as Variation and Distribution, Bionomics, Embryology and 

 Larval Stage, and Anatomy. In present speculative and theo- 

 retical suggestions — and they are valuable and stimulating — 

 such records as these, referring to animal life not usually dealt 

 with in evolutionary discussions, are of the very utmost import- 

 ance, and will be found to be both qualitative and confirmatory 

 on many disputed views. 



Colour variation appears to be most striking in Nudibranchs. 

 The range of colours comprises red, yellow, pink, brown, and 

 grey of many shades, purple, slatey-blue, sage-green, and perhaps 

 others. "In a series of specimens received from the Isle of 

 Man slate-colour and greyish-blue predominate. At Plymouth 



red and yellow, though not universal, are very common 



At Plymouth many marine animals as well as seaweeds are 

 reddish." Both adult and young Nudibranchs appear and dis- 

 appear suddenly in considerable quantities, especially before 

 spawning and after hatching. Alder and Hancock were disposed 

 to disbelieve in these migrations, but Sir Charles Eliot considers 

 " there is a good deal of evidence for their occurrence. It is 

 not known how they are performed, but as creeping is an 

 extremely slow process, it is possible that the animals make use 

 of currents in which to suspend themselves." As regards pro- 

 tective resemblance, Nudibranchs frequently escape notice be- 

 cause their colour and to some extent their shape fit in with 

 their surroundings, and, as Sir Charles Eliot remarks, " Of the 

 reality of this phenomenon no one can doubt who has collected 

 them in the tropics." But he also qualifies some suggestions on 

 this subject. He says : — 



" These resemblances are striking, and clearly cannot be dis- 

 advantageous to the molluscs, but still I think that much of the 

 language used about protective coloration, as if the animals were 

 made to look like their surroundings by some special power either in 

 themselves or external, is too anthropomorphic. It underestimates 

 the importance of one commonplace factor in the situation, namely, 

 that the two objects which resemble one another (such as the Nudi- 



