300 SINGLE CENTRES OF CREATION. [CHAP. XXI. 



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 afterwards isolated country, they would be little liable 

 to modification ; for neither migration nor isolation in themselves 

 effect anything. 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 proceeded from the same source, 

 as they are descended from the same progenitor. In the case of 

 those species which have undergone during whole geological 

 periods little modification, there is not much difficulty in believing 

 that they have migrated from the same region ; for during the 

 vast geographical and climatal 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 now 

 inhabiting distant and isolated regions, must have proceeded 

 from one spot, where their parents were first produced : for, as 

 has been explained, it is incredible that individuals identically 

 the same should have been produced nx/m parents specifically 

 distinct. 



Single Centres of supposed Creation. We are thus brought to 

 the question which has been largely discussed by naturalists, 

 namely, whether species have been created at one or more points 

 of the earth's surface. Undoubtedly there are many cases of 

 extreme difficulty in understanding how the same species could 

 possibly have migrated from some one point to the several distant 

 and isolated points, w-here now found. Nevertheless the simplicity 

 of the view that each species was first produced within a single 

 region captivates the mind. He who rejects it, rejects the vera 

 causa of ordinary generation with subsequent migration, and calls 

 in the agency of a miracle. It is universally admitted, that in 

 most cases the area inhabited by a species is continuous ; and 

 that when a plant or animal inhabits two points so distant from 

 each other, or with an interval of such a nature, that the space 

 could not have been easily passed over by migration, the fact is 

 given as something remarkable and exceptional. The incapacity 

 of migrating across a wide sea is more clear in the case of terres- 



