Gradualism
Charles Darwin was influenced by Lyell's Principles of Geology. Everything we see from the past is explained by processes we can see going on today. The present is key to the past. So Darwin thought evolutionary must occur gradually, not in jumps ('saltations'), since we do not see big changes going on today. Extreme and rapid changes in an organism would be most likely to be fatal. It is almost universally agreed that complex organs such as the eye must have evolved by many small steps rather than by one or two mutations.