Organic Evolution. 181 



called trigger. These two are so hinged to the underlying 

 interspinous bones and so related to each other that, when 

 once the defensive spine in front is erected, it cannot be 

 forced down until the trigger is lowered. The second 

 example of special adaptation is well displayed in specimens 

 of the mud-tortoise Trionyx. Between the last vertebra 

 of the neck and the first fixed vertebra of the dorsal series 

 is a beautiful hinge-joint, enabling the neck to be bent 

 back, S-fashion, when the creature withdraws its head 

 within the carapace. These are only one or two particular 

 instances of what any one who will visit the National 

 Museum may see for himself admirably displayed and 

 illustrated. 



No one can, one would suppose, pass through the 

 galleries in Cromwell Eoad and remain quite insensible to 

 the beauties of animal life. Beauty of form and beauty of 

 colour are conspicuously combined in many species of birds 

 and insects. And much of this colour-beauty and splendid 

 iridescence is known to be due to minute scales, to thin 

 films of air or fluid, and to microscopically fine lines 

 developed upon scales or feathers. But there is one phase 

 of beauty which cannot be exhibited in the museum the 

 beauty that comes of life as opposed to death. For this 

 we must go out into the free air of nature, where the 

 animals not only have lived, but are still instinct with the 

 glow of life, and where the silence of the museum galleries 

 is replaced by the song of birds and the hum of insect- 

 wings. 



How have this wealth, this diversity, this beauty, this 

 manifold activity, which we summarize under the term 

 " animal life," been produced ? 



If we answer this question in a word the word " evolu- 

 tion " * we must remember that this word merely ex- 

 presses our belief in a general fact ; and we must not 



* It is beyond the scope of this book to give tlie evidences of evolution. 

 Such evidence from embryology, from distribution, and from palaeontology, 

 is now. abundant. For palseontological evidence, see Nicholson's " Manual 

 of Palaeontology," 3rd edit., especially the second volume on " Vertebrates," 

 by K. Lydekker. 



