[2] In 1853, after reading Charles Darwin's The Voyage of the Beagle and Hugh Miller's The Footprints of the Creator, Gulick presented his paper, "The Distribution of Plants and Animals", to the Punahou School Debating Society.

In 1855, he enrolled for one year at New York University and then Williams College in Massachusetts, and studied in their Lyceum of Natural History.

[6] That paper was "On Diversity of Evolution Under One Set of External Conditions", which was published in the Journal of the Linnean Society of London, Zoology in 1873.

In 1891, another paper, "Intensive Segregation, or Divergence Through Independent Transformation" was published in the Journal of the Linnean Society of London, Zoology.

[4][9] Later in 1905, he returned to Hawaii and sold his shell collection to Charles Montague Cooke Jr. the new curator of the Bernice P. Bishop Museum.

In 1872, Gulick was the first to propose the theory that the majority of evolutionary changes are the result of chance variation, which has no effect on the survival and reproductive success of a species (today called "genetic drift").

He came to this theory while noting that there was a large diversity of local populations of Hawaiian land snails (Achatinella) which showed random variation under seemingly identical environmental conditions.

Although he certainly promoted the importance of random factors in evolution, he also was a strong supporter of Darwinian natural selection, and this led to disagreement with Moritz Wagner's "Migration Theory" of the origin of species.

Romanes said of Gulick:...to his essays on the subject I attribute a higher value than to any other work in the field of Darwinian thought since the date of Darwin's death.

Some modern observers attribute the extinction of many endemic Hawaiian snail species to him and fellow collectors such as his schoolmate David Dwight Baldwin.