That although the giant sloth was an extinct animal, it resembled modern-day sloths in Argentina. This observation led him to consider that the modern-day animals might be descendants of the ancient giant sloths.

Malthus wrote about factors (such as hunger) that would limit the growth of population. Darwin realized that individual organisms must therefore compete for access to resources, and that any small variation that gave an individual an advantage would lead to its success over others. If the variations were successful, then individuals with them would survive and reproduce to a greater extent than individuals without these successful variants. Over time, these variations, or traits, would become more common in the population.

Wallace focused on Malthus’s writings about disease, and how disease limited the growth of populations. As he himself was suffering from malaria, Wallace realized that disease would eliminate the weakest members of a population, leaving the strongest (the most fit) individuals to survive and reproduce. He reasoned that this would lead to changes leading to adaptations, and even to new species.

Both Darwin and Wallace had “aha!” moments inspired by observations while on their voyages. Darwin had read Lyell’s work, and when he observed a bed of seashells in a cliff well above sea level and the effects of an earthquake he truly appreciated how much the geology of Earth changed. His observation of the extinct sloth and smaller but similar modern sloths led him to consider how resemblances suggested ancestral relationships. Wallace’s observations of distinct yet similar species on either side of a physical separation (e.g., a river or canyon) helped him think about his “closely allied” species and led him to the idea each species was somehow related to a pre-existing species. Without actually seeing these fossils, organisms, and events, it is less likely that either would have been able to develop his understanding of evolution.