Inferring and Explaining

76 in any species would have slight advantages (and others would have slight disadvantages) in sur- viving long enough to reproduce. Nature, itself, would be selecting. Owing to this struggle for life, any variation, how- ever slight and from whatever cause proceeding, if it be in any degree proftable to an individual of any species, in its infnitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its ofspring. Te ofspring, also, will thus have a better chance of surviving, for, of the many individuals of any spe- cies which are periodically born, but a small num- ber can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. 24 InferrIng and exPlaInIng e 10 . No two individuals are exactly the same; rather, every population displays enormous variability. 25 e 11 . Much of this variation is heritable. 26 t 2 0 . Survival in the struggle for existence is not random but depends in part on the heredity of the surviving individuals. This unequal survival constitutes a process of natural selection. 27 From t 2 0 , Darwinmakes onemore inference that gives him the title for his book and the explana- tion of descent with modifcation. t 3 0 . Over generations, this process of natural selection will lead to a continuing gradual change of populations—that is, to evolution and to the production of new species. 28 One Long Argument Darwin called On the Origin of Species “one long argument.” I have stressed that he actually defended two, quite distinct theories, but at the same time, it’s easy to see the truth in the one long argument characterization. Having clearly pre- sented the evidence for descent with modifca- tion, he then (actually, this is not the progression in the book) lays out the case for natural selection. e 1 . The earth ismuch older than had been previ- ously believed—thousands of millions of years. e 2 . The fossil record e 3 . The natural system e 4 . Patterns of geographical distribution e 5 . Morphological commonalities e 6 . Embryological oddities t 0 . Descent with modifcation e 7 . All species have great potential fertility. e 8 . Populations normally display stability. e 9 . Natural resources are limited. t 1 0 . Fierce struggle for existence—more are born than will reproduce e 10 . No two individuals are exactly the same. e 11 . Much of this variation is heritable. t 2 0 . Survival in the struggle for existence is not completely random; it is the product of natural selection. t 3 0 . Over generations, this process of natural selection will lead to evolution and to the production of new species. As always, rival explanations are possible. Almost every biologist now accepts t 0 , t 1 0 , and