v CHANGE FROM A REPTILE TO A BIRD 55 



heredity and variation. In form and character the off- 

 spring take after their parents, but in almost every case 

 there is some slight discernible difference. The in- 

 dividuals that have variations that fit them better for 

 life survive, those that have injurious, or, in some 

 cases, those that have only useless variations or none 

 at all, perish. Thus are produced new species, one 

 useful variation after another being accumulated by 

 Natural Selection. The sickly and those who are 

 unsuited to their surroundings have no chance, for 

 the law is mercifully ruthless. 



The facts that I have stated seem to prove much. 

 The struggle for existence is indisputable, and evolu- 

 tion through Natural Selection seems almost beyond 

 dispute. But when we come to investigate more in 

 detail how it has acted, we are met with great diffi- 

 culties. A living man of science has said that for the 

 explanation of the brilliant colours of the butterfly 

 Darwin's theory is but a barren formula. It may 

 be that only his own imagination is barren. Later 

 on, in a chapter on colour and song, I hope to 

 show that colour is, at any rate, connected with 

 Natural Selection. The particular difficulty that 

 confronts us when we try to trace the evolution of 

 birds is that the development of wings would have 

 been useless, and worse than useless, unless accom- 

 panied by other changes. And just as he who builds 

 a Latin verse conceives a master stroke and puts into 

 his line, that before was tame and commonplace, a 

 piirpurens pannus, then suddenly, to his dismay, finds 

 that his brilliant emendation has ruined the grammar 

 and the sense, so we may imagine a reptile, that 



