174 THE PRINCIPLES OF HEREDITY 



to suppose that the different varieties in each type had a 

 common origin in one species from which they differentiated 

 in various directions. Thus we attribute to cattle or dogs a 

 common origin in a single centre from a single ancestral type. 

 We do not suppose that they originated in diverse centres 

 from diverse types which afterwards fused more or less. 1 

 The mere fact that the varieties are able to interbreed shows 

 the community of origin. Indeed, while all nature affords 

 evidence of the constant tendency towards differentiation, we 

 know no instance of real approximation between diverse 

 forms, 2 nor do we know of a single instance of the same form 

 originating in diverse centres. We must conclude, therefore, 

 that it is in the highest degree probable that each disease 

 arose in a single centre, whence it spread till it reached its 

 present limits. 



293. As in the case of other forms of life, we have every 

 reason to suppose that the human race arose in a single 

 centre from a single ancestral type. All our information 

 indicates the Eastern Hemisphere as the place of origin. 

 Parts of it have been inhabited by a dense and settled 

 population from a time immensely remote. " Behind dim 

 empires ghosts of dimmer empires loom." Beyond the traces 

 of the oldest empires we find traces of primitive agricultural 

 communities, and far beyond these the remains of the cave- 

 men and hunters of the Stone Age. Even a race of hunters 

 tends to increase faster than the food supply. Doubtless the 

 pressure of population in the Old World led to the coloniza- 

 tion of the New. But even in the New World there are 

 signs of a civilization so ancient that some authorities have 

 placed its beginnings as far back as a score or more of 

 thousands of years. Now, with the exception of malaria, it 

 is extremely doubtful whether any zymotic disease existed in 

 the whole of the New World at the time of its discovery by 



1 It is very probable, as Darwin supposed (Animals and Plants, vol. i., 

 pp. 34-85), that domesticated dogs and cattle have descended from 

 several wild varieties, but his own labours have rendered it clear that 

 all the wild varieties were descended from a common ancestral form. 

 Moreover, among them, even under domestication, there has been much 

 more differentiation than approximation. It is possible, of course, for 

 closely-related varieties to interbreed and so fuse into a single variety, 

 but this rarely if ever happens except in the case of domesticated animals. 

 No one has ever maintained that unrelated forms of micro-organisms 

 interbreed. 



2 Thus, though the whale has mimicked the fish, though certain 

 species of insects have mimicked other species, the resemblance is in 

 outward form only. The internal structural differences remain as great 

 as ever. 



