62 ISLAIN-D LIFE 



PART I 



to genera rather than to single species, it will be well here 

 to explain what is meant by a genus, and how genera are 

 supposed to arise. 



A genus is a group of allied species which differs from 

 all other groups in some well marked characters, usually of 

 a structural rather than a superficial nature. Species of 

 one genus usually differ from each other in size, in colour 

 or marking, in the proportions of the limbs or other organs, 

 and in the form and size of such superficial appendages as 

 horns, crests, manes, &c. ; but they generally agree in the 

 form and structure of important organs, as the teeth, the 

 bill, the feet, and the wings. When two groups of species 

 differ from each other constantly in one or more of these 

 latter particulars they are said to belong to different genera. 

 We have already seen that species vary in these more 

 important as well as in the more superficial characters. 

 If, then, in any part of the area occupied by a species some 

 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 become fixed in the 

 proportions most beneficial to the animal, we shall have the 

 first species of a new genus. 



A creature which has been thus modified in important 

 characters will be a new type, specially adapted to fill a 

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

 tainly have arisen from an extensive or dominant species, 

 because only such are sufficiently rich in individuals to 

 afford an ample supply of the necessary variations, and it 

 will inherit the vigour of constitution and adaptability to 

 a wide range of conditions which gave success to its 

 ancestors. It will therefore have every chance in its favour 

 in the struggle for existence; it may spread widely and 

 displace many of its nearest allies, and in doing so will 

 itself become modified superficially and become the parent 

 of a number of subordinate species. It will now have 

 become a dominant genus, occupying an entire continent, 

 or perhaps even two or more continents, spreading in every 

 direction till it comes in contact with competing forms 

 better adapted to the different environments. Such a 

 genus may continue to exist during long geological 



