Theory of Organic Evolution:

The correct answer is option 'C', the Theory of Organic Evolution. This theory, also known as the theory of evolution, is based on the concept of change with descent. It explains how species change over time and how new species arise.

Charles Darwin is the most prominent figure associated with the theory of organic evolution. He proposed the concept of natural selection as the driving force behind the process of evolution. According to Darwin, individuals within a population vary in their traits, and those with traits that are advantageous for survival and reproduction are more likely to pass on their genes to the next generation. Over time, this leads to the accumulation of beneficial traits in a population and the emergence of new species.

Let's break down the key points of the theory of organic evolution:

1. Change: The theory of organic evolution recognizes that change is a fundamental characteristic of living organisms. Species are not fixed entities but rather undergo modifications over time.

2. Descent: The concept of descent refers to the idea that new species arise through the modification of pre-existing species. This means that all organisms are related through a common ancestry.

3. Variation: Within a population, individuals show variation in their traits. This variation is a result of genetic mutations, genetic recombination, and other factors.

4. Natural Selection: The theory proposes that the environment acts as a selective force, favoring individuals with traits that enhance their survival and reproductive success. These individuals are more likely to pass on their genes to the next generation, leading to the gradual adaptation of the population to its environment.

5. Speciation: Over long periods of time, the accumulation of beneficial traits can result in the formation of new species. This process, known as speciation, occurs when populations become reproductively isolated from each other and diverge in their characteristics.

In conclusion, the theory of organic evolution explains how species change over time through the process of descent with modification. It encompasses the concepts of variation, natural selection, and speciation. This theory has revolutionized our understanding of the diversity of life on Earth and provides a framework for studying the relationships between different species.