176 THE LESSON OF EVOLUTION 



necessarily a progressive principle ; it only forces 

 organisms to become as perfectly adapted as possible 

 to the conditions, and then keeps them there. The 

 only difficulty is with the useless characters just 

 mentioned. But here also the difficulty is not so 

 formidable as it looks. Most of these changes are 

 colour changes, and these necessarily have a limit. 

 The change of a red corolla into a blue one need not 

 take long, and the change ends. Useless changes in 

 the size or shape of an organ are probably limited 

 by natural selection, for they might easily become 

 disadvantageous. Such is the case with the hind 

 claw of the sky-lark, which has grown long and 

 straight simply because the bird has given up perch- 

 ing on trees. The horns in male beetles are also 

 limited from the same cause. The sculpture on the 

 elytra of Coleoptera and Hemiptera seems to be a 

 difficulty, for further elaborations would not be 

 disadvantageous ; and yet these characters are as 

 constant as others. Many other difficulties could be 

 cited, and I think that this is a question on which 

 no definite opinion can as yet be given. 



Physical Isolation. Since the publication of 

 Moritz Wagner's paper " On the Law of the 

 Migration of Organisms," 49 geographical isolation is 

 so well known that it is quite unnecessary for me to 

 explain it. I will, instead, illustrate it with 

 examples from New Zealand. 



The crow (Glaucopis) of the North Island has its 

 wattles coloured blue, while those of the South 



"English trans, published by E. Stanford, London, 1873. 



