Darwin was heavily influenced by the work of Thomas Malthus, who argued that human populations tend to grow faster than their food supply, leading to competition for resources and a struggle for survival. This concept of "struggle for existence" was crucial for Darwin in formulating his theory of natural selection.

Here's how Darwin applied Malthus' ideas:

1. From Humans to All Organisms: While Malthus focused on humans, Darwin extended the concept to all living organisms. He realized that all populations have the potential to reproduce at a rate that would outstrip their resources, leading to competition for food, shelter, mates, and other necessities.

2. Variation and Heritability: Darwin observed that within any population, individuals exhibit variation in their traits. He also knew that some of these traits were heritable, passed down from parents to offspring.

3. Natural Selection: Darwin combined these ideas to propose that individuals with traits that make them better suited to their environment are more likely to survive and reproduce, passing those advantageous traits onto their offspring. This process, known as natural selection, drives the gradual change in populations over generations, leading to the evolution of new species.

Example: Imagine a population of birds with varying beak sizes. If a drought occurs, making seeds harder to crack, birds with larger, stronger beaks will be better equipped to survive and reproduce. Over time, the population will shift towards having a higher proportion of birds with larger beaks, because those with smaller beaks are less likely to survive and pass on their genes.

In summary: Darwin's theory of natural selection was heavily inspired by Malthus' ideas about population growth and resource limitation. He expanded this concept to encompass all organisms, recognizing that variation within populations and the inheritance of traits provide the raw material for natural selection to act upon, ultimately driving evolution.