THE ORIGIN OF SPECIES AND GENERA 299 



How Genera Originate. 



Let us now see how the same principles will explain 

 the origin of genera. A genus is a group of allied species 

 which differs from all other groups in some well-marked 

 characters of a structural rather than of a sujMrficial 

 nature. For example, species of the same genus usually 

 differ from each other in size, in colour or markings, in 

 the proportion of the limbs or other organs, and in the 

 form and size of such superficial appendages as horns, 

 crests, manes, and the accessory ornamental plumes of 

 birds ; but they generally agree in the form and structure 

 of important organs, as the teeth, the bill, the feet, and 

 the wings. But when two groups of species differ from 

 each other constantly, and to a w^ell-marked degree, in 

 one or more of these latter characters, they are said to 

 belong to distinct genera ; and we have seen that species 

 vary in these as well as the more superficial characters — 

 the bill, the feet, and the wangs varying in size and pro- 

 portions just as frequently as do the colours or the orna- 

 mental parts of the plumage. If then, in any portion of 

 the area occupied by a species, some important change of 

 habits becomes useful to it, all such structural variations 

 as facilitate the change will be accumulated by natural 

 selection, and when they have thus acquired the pro- 

 portions most beneficial under the altered conditions, we 

 shall have the first species of a new genus. 



A creature which has been thus modified in important 

 characters will form a new type, specially adapted to fill 

 its place in the economy of nature. It will almost cer- 

 tainly have arisen from an extensive or dominant group, 

 because such only are sufficiently rich in individuals to 

 afford an ample supply of the needful variations, and it 

 will therefore inherit the vigour of constitution and 

 adaptability to a wide range of conditions which gave 

 success to its ancestors. It will thus have every chance 

 in its favour in the struggle for existence; it may 

 spread widely and displace some of its nearest allies, and 

 in doing this will extend into new areas, where it will be 



