VI.] SPECIES AND TIME. 153 



these changes occur at distant intervals, but it must be 

 borne in mind that in tracing back an animal to a remote ^ 

 ancestry, we pass through modifications of such rapidly-in- J 

 creasing number and importance that a geometrical pro- ' 

 gression can alone indicate the increase of periods which 

 such profound alterations would require for their evolution 

 through " Natural Selection " only. 



Thus let us take for an example the proboscis monkey 

 of Borneo (Semnopithecus nasalis). According to Mr. Dar- 

 win's own opinion, this form might have been " sensibly 

 changed " in the course of two or three centuries. Accord- 

 ing to this, to evolve it as a true and perfect species one 

 thousand years would be a very moderate period. Let 

 ten thousand years be taken to represent approximately the 

 period of substantially constant conditions, during which 

 no considerable change would be brought about. Now, it 

 one thousand years may represent the period required for 

 the evolution of the species S. nasalis, and of the other 

 species of the genus Semnopithecus, ten times that period 

 should, I think, be allowed for the differentiation of that 

 genus, the African Cercopithecus, and the other genera of 

 the family Simiidae the differences between the genera 

 being certainly more than tenfold greater than those 

 between the species of the same genus. Again, we may 

 perhaps interpose a period of ten thousand years' com- 

 parative repose. 



For the differentiation of the families Simiidae and 

 Cebidae so very much more distinct and different than any 

 two genera of either family a period ten times greater 

 should, I believe, be allowed than that required for the 

 evolution of the subordinate groups. A similarly increasing 

 ratio should be granted for the successive developments of 

 the difference between the Lemuroid and the higher forms 

 of primates ; for those between the original primate and 

 other root-forms of placental mammals ; for those between 



