Varieties of species that differ in one observable way may consistently differ in another way too. These are observable correlations, and the hidden laws that dictate their occurrence, Darwin notes, are unknown.

Darwin employs this principle to explain how the minute differences of newly propagated varieties transmute the wide variability between incipient species. The most thoroughly diversified animals or plants tend to become even more so because they outcompete their less diverse ones. Divergence, therefore, builds on divergence, leading to even greater variety and the transmutation of species.

Although he states that it’s not a precise definition, Darwin refers to instinct as unconscious actions by animals that would require a learning experience for humans to do. Instincts are generally observable from a young age. The animal, Darwin writes, acts on instinct “without knowing for what purpose” (699).