Makes sense to put it here then everyone can contribute.
Darwin Chapter IX
In this chapter Darwin shows that the difference between varieties and separate species is simply a matter of degree. If we define separate species as being unable to form fertile offspring then, of course, separate species will be unable to form fertile offspring. But in practice it is often totally impossible to define whether two populations are separate species or merely varieties.
Darwin's objective was to prove that with time varieties evolve into species. As an extension it is obvious that species evolve into genera, etc. His case suffers a little in that at the time he was unaware that the infertility that arises through inbreeding springs from a different cause to the infertility that develops between species. This led him to a few dead ends but, read in the light of what we now know, his examples are still very interesting.
And we now know that the gap between species can change with changing environment. For example two species of Galapagos finches readily form hybrids during optimum environmental conditions but separate when times are hard. It seems even human groups behave the same way. John Dewey claimed as long ago as 1915 that different classes and social groups mix with each other more during expansive econmomic periods. It's probable that ancient human groups obeyed this same rule.
Anyone want to add anything, take it away or dismiss it totally? Or try another chapter?
