Q-maRAPBIOAL niSTRlBUriON. 877 



they spread out iuto new countries. In their new homes 

 they will be exposed to new conditions, and will frequently 

 undergo further modification and improvement; and thus 

 they will become still further victorious, and will produce 

 groups of modified descendants. Qn this principle of ift- 

 heritance with modification we can understand how it is 

 that sections of genei'a, whole genera, and even families, 

 are confined to the same areas, as is so commonly and noto- 

 riously the case. 



There is no evidence, as was remarked in the last 

 chapter, of the existence of any law of necessary develop- 

 ment As the variability of each species is an independent 

 property, and will be taken advantage of by natural selec- 

 tion, only so far as it profits each individual in its complex 

 struggle for life, so the amount of modification in different 

 species will be no uniform quantity. If a number of species, 

 after having long competed with each other in their old 

 home, were to migrate in a body into a new and afterward 

 isolated country, they would be little liable to modification; 

 for neither migration nor isolation in themselves effect any 

 thing. These principles come into play only by bringing 

 organisms into new relations with each other and in a lesser 

 degree with the surrounding physical conditions. As we 

 have seen in the last chapter that some forms have retained 

 nearly the same character from an enormously remote 

 geological period, so certain species have migrated over 

 vast spaces, and have not become greatly or at all modified. 



According to these views, it is obvious that the several 

 species of the same genus, though inhabiting the most 

 distant quarters of the world, must originally have pro- 

 ceeded from the same source, as they are descended 

 from the same progenitor. In the case of those 

 species which ha,ve undergone, during whole geological 

 periods, little modification, there is not much difficulty in 

 believing that they have migrated from the same region; 

 for daring the vast geographical and cliroatical changes 

 which have supervened since ancient times, almost any 

 amount of migration is possible. But in many other cases, 

 in which we have reason to believe that the species of a 

 genus have been produced within comparatively recent 

 times, there is" great 'difficulty on this head. It is also 

 obvious that the individuals of the same species, though 



