Darwin Assumed that evolutionary change was always slow and gradual.

Charles Darwin did propose that evolutionary change often occurred through a slow and gradual process, but he also recognized that there could be periods of more rapid change. This concept is known as "gradualism" and was a key component of Darwin's theory of evolution. However, it is important to note that Darwin's understanding of the mechanisms of inheritance and genetic variation was limited compared to modern knowledge.

To understand why Darwin assumed that evolutionary change was often slow and gradual, we can look into his observations and the scientific understanding of his time. Darwin spent many years studying diverse species, particularly during his voyage on the HMS Beagle. He noticed that living organisms often exhibited a range of variations within a population. He proposed that these variations arise due to random alterations in heritable traits, which he called "individual variation."

Darwin also recognized that the environment plays a crucial role in shaping the survival and reproductive success of individuals with certain traits. Through the process of natural selection, individuals with advantageous variations have a higher likelihood of surviving and passing on their traits to the next generation. Over time, these small, advantageous changes can accumulate and lead to the formation of new species.

Based on these observations, Darwin theorized that evolution typically occurs through the gradual accumulation of small changes over long periods of time. He suggested that the slow pace of evolution was due to the slow rate at which favorable variations might arise and spread within a population. Additionally, the fossil record available during Darwin's time showed a relatively smooth transition from one form of life to another, further supporting his idea of gradualism.

However, it is essential to note that Darwin's assumption of gradualism was not absolute. He recognized that some organisms could experience more rapid changes due to various factors, such as sudden changes in the environment or migrations to new habitats. He even proposed the concept of "punctuated equilibrium," suggesting that evolutionary change may occur in rapid bursts followed by long periods of stability.

In summary, while Charles Darwin assumed that evolutionary change was often slow and gradual, he also acknowledged exceptions to this pattern. Modern understanding of evolution has expanded upon Darwin's ideas and recognizes that the pace of evolutionary change can vary, depending on several factors such as the presence of strong selective pressures or genetic mechanisms that allow for rapid adaptation.