278 ANIMAL DEFENCES 



arrangements now to be described are much less effective than 

 might at first sight be imagined. Aggressive animals would die 

 out altogether if their prey gradually evolved into a perfectly 

 protected condition, and such a state of things can never be 

 realized. Suppose a species A to feed upon other species B, c, 

 and D. Then as these develop characteristics which make them 

 more and more difficult to detect, A will keep pace more or less 

 with this by improvement of its sense organs and powers of 

 observation. Besides which, it does not follow that a protected 

 form which deceives the eye will necessarily deceive other senses, 

 such, e.g., as that of smell. This, however, is a matter which still 

 requires working out, for most attention has so far been paid to 

 arrangements which may metaphorically be called " optical 

 illusions ". And again, even if B, c, and D succeeded in evading 

 the pressing attentions of A altogether, that species might be 

 enabled to divert its energies in some other direction. 



PROTECTIVE RESEMBLANCE (as arrangements tending to incon- 

 spicuousness may be called) may either be general, so as to 

 harmonize with the colour scheme, illumination, &c., of the sur- 

 roundings, or they may be special, so as to bring about an 

 imitation of some particular object, as a twig or leaf. It will 

 be convenient to consider these cases separately. 



GENERAL PROTECTIVE RESEMBLANCE 



TRANSPARENCY. Vast numbers of marine animals belonging 

 to many different groups are to be found either swimming or 

 drifting in the surface layer of the sea, and it is characteristic of 

 such forms that they should be of glass-like transparency, which 

 obviously makes them very inconspicuous. Such an arrange- 

 ment, of course, only partly meets the situation, for the internal 

 organs of these "glass animals", to use the German expres- 

 sion, must of necessity be at least partially opaque, and many 

 of them may be condensed as it were into a limited region of 

 the body. But such organs, after all, look like bits of floating 

 weed. Good examples are furnished by free-swimming Ascidians, 

 such as Salps, many pelagic members of the Sea- Snail kind 

 (Gastropods), especially those which are known as Sea- Butterflies 

 (Pteropods) and Swimming- Snails (Heteropods), many Crustacea, 

 numerous Worms, and above all creatures such as Jelly -Fish 

 (Hydrozoa), together with the various species of Comb -Jellies 



