60 CONTACTS WITH DARWINISM IV [ch. vi 



rarely do the really large families show a single genus with the 

 range of the whole family, a feature very common in small and 

 frequent in medium-sized families, and as still more rarely does 

 that genus in a large family show a species with the whole range 

 of the genus, it is clear that any adaptation responsible for wide 

 spread must be generic. What is much more probably the case, 

 inasmuch as these widespread genera are admittedly of simple 

 rather than of complex type, is that the parent of the genus still 

 possesses great adaptability, or suitability to a considerable range 

 of conditions. This will enable it to move far with less difficulty 

 than usual, and as at the same time its structural evolution, which 

 has probably little or no relation to adaptation, will be going on, 

 it will give rise to more and more species. These will probably 

 inherit their parent's general suitability to conditions, but it is 

 quite probable that it may all the time be getting less (perhaps at 

 each mutation), so that each new species may be liable to become 

 more localised than its predecessor in regard to the total range 

 possible to it, while at any given time it will of course be more 

 local on account of its greater youth. 



