APPENDIX. 401 
so as to harmonize with our hypothesis of descent. The geo- 
graphical barriers to diffusion (mountains, deserts, rivers, straits, 
etc.), have not been taken into consideration in this general 
sketch of migration, because, in earlier periods of the earth’s 
history, they were quite different in size and form from what 
they are to-day. The gradual transmutation of catarrhine apes 
into pithecoid men probably took place in the tertiary period in 
the hypothetical Lemuria, and the boundaries and forms of the 
present continents and oceans must then have been completely 
different from what they are now. Moreover, the mighty in- 
fluence of the ice period is of great importance in the question 
of the migration and diffusion of the human species, although 
it as yet cannot be more accurately defined in detail. I here, 
therefore, as in my other hypotheses of development, expressly 
guard myself against any dogmatic interpretation; they are 
nothing but first attempts. 
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