How would early geologists such as William Smith explain the observation that certain species were found only in small number of rock layers while others were found in many different layers?

William Smith (1769–1839), known as the "Father of English Geology," was an English geologist credited with creating the first nationwide geological map. He made several important contributions to the field, including the principle of faunal succession, which is key to understanding the question posed.

If early geologists like William Smith were to explain the observation that certain species were found only in a small number of rock layers while others were found in many different layers, they would likely use the principle of faunal succession. This principle states that sedimentary rock layers contain fossilized flora and fauna that succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances.

Smith would argue that the species found only in a small number of rock layers were likely to have lived during a relatively short period of geological time. Their limited occurrence in the geological record would indicate that these species had a brief range in time before they became extinct or evolved into different forms. This could be due to a variety of factors, such as rapid environmental changes, local ecological conditions, or evolutionary events that led to the emergence of new species.

On the other hand, species found in many different layers would be considered to have had a longer duration in geological time. These species were more widespread and existed over multiple geological periods. Their fossils would be found in a variety of rock layers because they were successful in adapting to changing environments or were widespread geographically.

Smith's observations and interpretations were based on the principle of faunal succession and his extensive work in the field, where he meticulously documented the vertical and horizontal distribution of fossils in the sedimentary rock layers of England. His work laid the foundation for biostratigraphy, which is the branch of stratigraphy that uses fossil organisms to correlate and date rock layers.

In summary, early geologists like William Smith would explain the limited occurrence of certain species in the rock record as indicative of their short-lived nature in geological terms, while the widespread presence of other species in many layers would reflect their longer existence and adaptability over time.