382 RECAPITULATION. [CHAP. XV. 



cessive generations have travelled from some one point to all the 

 others. We are often wholly unable even to conjecture how this 

 could have been effected. Yet, as we have reason to believe that 

 some species have retained the same specific form for very long 

 periods of time, immensely long as measured by years, too much 

 stress ought not to be laid on the occasional wide diffusion of the 

 same species ; for during very long periods there will always have 

 been a good chance for wide migration by many means. A broken 

 or interrupted range may often be accounted for by the extinction 

 of the species in the intermediate regions. It cannot be denied 

 that we are as yet very ignorant as to the full extent of the various 

 climatal and geographical changes which have affected the earth 

 during modern periods ; and such changes will often have facili- 

 tated migration. As an example, I have attempted to show how 

 potent has been the influence of the Glacial period on the distri- 

 bution of the same and of allied species throughout the world. 

 We are as yet profoundly ignorant of the many occasional means 

 of transport. With respect to distinct species of the same genus 

 inhabiting distant and isolated regions, as the process of modifica- 

 tion has necessarily been slow, all the means of migration will 

 have been possible during a very long period ; and consequently 

 the difficulty of the wide diffusion of the species of the same genus 

 is in some degree lessened. 



As according to the theory of natural selection an interminable 

 number of intermediate forms must have existed, linking together 

 all the species in each group by gradations as fine as are our exist- 

 ing varieties, it may be asked, Why do we not see these linking 

 forms all around us? Why are not all organic beings blended 

 together in an inextricable chaos 1 With respect to existing forms, 

 we should remember that we have no right to expect (excepting 

 in rare cases) to discover directly connecting links between them, 

 but only between each and some extinct and supplanted form. 

 Even on a wide area, which has during a long period remained 

 continuous, and of which the climatic and other conditions of life 

 change insensibly in proceeding from a district occupied by one 

 species into another district occupied by a closely allied species, 

 we have no just right to expect often to find intermediate varieties 

 in the intermediate zones. For we have reason to believe that 

 only a few species of a genus ever undergo change ; the other 

 species becoming utterly extinct and leaving no modified progeny. 

 Of the species which do change, only a few within the same 

 country change at the same time ; and all modifications are slowly 

 effected. I have also shown that the intermediate varieties which 

 probably at first existed in the intermediate zones, would be liable 

 to be supplanted by the allied forms on either hand ; for the latter, 

 from existing in greater numbers, would generally be modified and 



