Resumo de On The Origin Of Species By Natural Selection

Early on in Darwin's first five-year voyage on the HMS Beagle, he observed that, despite the distances between the remote areas he visited, the varieties of flora and fauna he found were similar in structure and function. These led him to develop his idea that species were not immutable, but were forced to adapt to their ever-changing environments. In his introduction to the first edition of The Origin of Species, Darwin noted: "I was much struck with certain facts in the distribution of the (plant and animal) inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to throw some light on the origin of the species - that mystery of mysteries, as it has been called by one of our greatest philosophers." After over twenty years of further research, Darwin published his findings.

Like all scientists, Darwin built his theory upon those of his predecessors. However, scientific opinion was always and remains - somewhat divided as to what contribution the theory makes to the biological sciences. Throughout the book, Darwin openly admits to the possibility of error and the need for further investigation; he is careful to point out that the idea of evolution by natural selection is "one of long argument."

To comprehend the vast amount of information contained in the work, one must examine it in its entirety. Still, this sampling of chapter headings and brief content summaries may provide some general information.

Chapter II: Variation Under Nature
Variations within a species are indistinguishable at first, but gradually may develop into differences that can restrict one group's range or ability to obtain food or escape predators ... Thus, "varieties tend to become converted into new and distinct species ... and throughout nature the forms of life which are now dominant tend to become still more dominant by leaving any modified and dominant descendants."

Chapter III: Struggle for Existence
"... When a plant or animal is placed in a new country amongst new competitors, the conditions of its life will generally be changed in an essential manner....... If its average numbers are to increase...... we should have to give it some advantage over a different set of competitors or enemies." Each organic being is striving to multiply to be vigorous, healthy, and to survive - often at the expense of members of its own species or those of a competing species.

Chapter IV: Natural Selection; or the Survival of the Fittest
The "fitness" of a species is modified by several different processes. For example, sexual selection may occur when males of a population must compete with other males to possess mates. Those possessing some advantage -better weapons, greater energy, or more beautiful song or plummage - are more apt to survive or attract a mate, likely to leave the most progeny. Over time, such gradual adaptation, along with changing conditions and outside competition, can cause "an infinite diversity in structure, constitution, and habits, advantageous to one set of offspring over another, or to one variety within a species over another. "This principle of preservation, or the survival of the fittest, I have called Natural Selection.. . "

As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead n d broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.

Chapter V: Laws of Variation
Reproductive "chance" creates variations. When the conditions of a species alter, those individuals that survive to reproduce may have beneficial modifications - organs or limbs that either become stronger or more useful, or else, when not needed for survival, weakened and diminished. This is not to say, however, that the formation f organs serving little purpose does not occur; it most certainly does. The human appendix may exemplify just such a process phenomenon. But any variation within a species is, inevitably, a long, slow process.

Chapter VIII: Instinct
Habitual instincts are inherited within each species. Ants and bees build their nests and hives with no previous experience. Birds migrate and build homes according to their unique inner senses. But instincts too may change over time as "consequences of one general law leading to the advancement of all organic beings ... multiply, vary, let the strongest live and the weakest die."

Other chapters deal with related topics: hybridism; living species compared to those of ancient geological periods; extinction; geographical distribution of organisms; relationships between species; and the classification of organisms. Objections to the general theory of evolution are presented in both Darwin's conclusion and glossary of terms.

Darwin's observations led him to believe that species did adapt to their changing surroundings. Furthermore, he was led to defend as a logical, observable - and even religious - corollary of this conclusion, a theory advancing the probability of common descent for all living creatures.

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