In the task of building an over-all view of the world (which we might call, using the word loosely, the task of philosophy) one is perhaps drawing a line of best fit through points represented by all the fields of special study, including those of the sciences.
The expression "line of best fit" comes from statistics. Think of just two variables, to make things easy to plot on a piece of paper. Say, the relationship between grams of fat and total calories in an item of fast food. You could make fat grams the X axis and total calories the Y axis. Then represent each examined sort of fast food as a point on that graph defined by the two axes. a hamburger, a cheeseburger, a box of french fries, an ice cream cone without sprinkles, an ice cream cone with sprinkles, etc. Each gets just one point, defined by specialists.
Odds are good your graph won't result in any neat line. This is a "scattershot graph." But there will likely be a general common trend, that the points move diagonally upward and ouward -- i.e. fat correlates roughly with calories. So you produce a straight line summing up the data as best you can, keeping the variance between your line and the data points as small as possible.
That is the relation between philosophy and special studies of all sorts I have in mind. Of courtse, the results of the special sciences keep changing, as you may have noticed in terms of the news from CERN for example. So we might think of subatomic physics as it exists circa 2011 as one of the data points in our scattershot graph. If my philosophy is sound, over time the dots that are furthest out from my line of best fit will move toward it, not away. I need not pretend that all known points fit perfectly along my line -- if I make that claim, I'm surely delusional.
This idea of a line of best fit, though, captures well what Jacques Barzun is doing in Darwin, Marx, Wagner, when he argues with Darwin, and with contemporary biology insofar as it continues to reflect those aspects of Darwinism that stray furthest from the line he draws.
His own special science, the field where Barzun first made his initial scholarly reputation, is the history of music in the 19th century.
And Charles Darwin? Barzun prefers the insights of his grandfather, Erasmus Darwin. As it happens, Erasmus even wrote about the evolutionary significance of the opposable thumb, though he chose to do it in verse:
The hand, first gift of heaven! to man belongs;