What is Darwin's definition of 'natural selection'?

Charles Darwin studied life sciences in the mid to late 19th century, and his theory of how evolution progressed was based on natural selection. Through the study of life sciences, Darwin saw that over long periods of time, through the process of evolution, certain species and traits would continue to progress and reproduce while others would die off, not allowing their traits to be passed onto following generations.

Darwin saw this process as a natural selection, where whichever living organism that was able to adapt favorably to changing environments or conditions would be selected to continue to reproduce, passing on its favorable traits to the next generation.

If a living organism had unfavorable traits (for example- short legs, small eyes, or a genetic disorder) which prevented it from living successfully (acquiring nourishment and defending itself) then it would most likely die and not produce offspring with the same deficient characteristics. These being the species or traits NOT selected to continue.