XIII EVOLUTION AND DISTRIBUTION OF ANIMALS 269 



and would soon form a distinct group of individuals 

 breeding together and constituting a species — the first 

 species of the new genus. This species having in time 

 supplanted the parent species, and being better adapted 

 than it for the battle of life, would almost certainly cover 

 a wider area, and thus come into competition with several 

 of the allied species of the old genus, some of which it 

 would also probably supplant, and occupy the areas they 

 formerly occupied. But as the old genus had been modi- 

 fied into distinct sjKcies (differing, perhaps, slightly in 

 colour or habits in accordance with the varying physical 

 conditions), so the now widespread species of the new 

 genus would vary, and become modified in a somewhat 

 analogous manner, forming a genus consisting of several 

 species. Now, if the generic form thus produced was one 

 of great inherent vigour and adaptability, and if the 

 peculiarity of structure it possessed was of considerable 

 importance, it would become what Mr. Darwin terms a 

 dominant group : that is, it would spread widely over the 

 earth under various modified forms suited to the various 

 conditions it became subject to. At last it would reach 

 its maximum of development, and cease to spread further, 

 either owing to its inability to adapt itself to further 

 changes of climate, &c., or, what is more likely, from its 

 coming into competition with other dominant groups which 

 had in like manner spread from some other centres. 



Now, during all this time, which may be termed the 

 period of growth of the genus, its area will have been 

 almost necessarily continuous, and the areas occupied by 

 its several species (also continuous) will probably overlap 

 each other. But now commences its period of decay. 

 Other groups of the same or allied families have given 

 rise to varieties which have also become dominant species 

 and genera, which, under the somewhat changed physical 

 conditions that in time have come about, beat it in the 

 battle of life, and force it to retire step by step from the 

 vast area it had overrun. First one species and then 

 another will dwindle away and become finally extinct, and 

 by so doing will necessarily leave gaps in its area of dis- 

 tribution. This process going steadily on, the time will 



