The last 40 years of the 19th century must have been an extraordinarily exciting tune for anyone with an interest in biology. The publication of Darwin’s Origin of Species in 1859 gave naturalists, as they called themselves in those days, a powerful new interpretive tool. Suddenly the facts they had been patiently collecting for two centuries and more took on meaning, became intelligible.

The peacock’s tail and the elephant’s trunk were no longer strange prodigies or inexplicable expressions of an enigmatic Providence. They could now be explained as the result of natural processes that were still going on around us. Our knowledge of plants and animals and the ways they live in the world exploded, grew so fast that the naturalist, a person who might study mollusks today, orchids tomorrow, and birds the day after, became extinct. There was too much to know. Only the specialist could survive.

The late 19th century was also the golden age of imperialism, the time when Europeans and their American cousins used their superior technology (“We have got the Maxim gun, and they have not”) to slaughter large numbers of their fellow humans and grab most of the earth’s resources.

The growth of scientific knowledge and the growth of the power of white people were inextricably connected. Lewis and Clark collected an enormous amount of information on the plants, animals, and geology of the west, but their ultimate goal was to find out how the U.S. could occupy the Louisiana Purchase. Later expeditions to explore possible routes for the transcontinental railroad all carried scientists, and many of the leading figures in late-19th-century American ornithology were Army doctors posted to forts in the west. They shot birds while the rest of the garrison shot Indians.

Alfred Russel Wallace, who shared the discovery of natural selection with Darwin, made his living, and developed his theories, by traveling the tropics collecting specimens for rich natural-history enthusiasts back home.

In Thomas Belt, we have a man who combined both of these strains. He was a naturalist who studied meteorology and geology as well as botany and zoology. And he was a mining engineer who helped extract gold from what we now call the third world for shipment back to England. His book A Naturalist in Nicaragua (University of Chicago Press paperback, $12.95), originally published in 1874, provides alternating doses of enlightened scientific thought and observation and the most benighted racism.

The effect is startling. You read pages of careful, thoughtful commentary on the behavior of army ants, for example, and you start to think how pleasant it would be to visit a tropical forest with this man. And then you come across a sentence like: “Both the Negroes and Indians are decidedly inferior to the whites in intellect,” which leads into a long discussion of the exact nature of their inferiority. It’s like finding out that an old friend, someone you regard as like a brother, is actually a member of the Klan.

Belt went to Nicaragua in February 1868 to begin a four-year stay. He was employed as a mining engineer by the Chontales Mining Company, which was extracting gold from veins exposed near the town of Santo Domingo (he calls it San Domingo). Santo Domingo is on the Atlantic side of Nicaragua east of the Lago de Nicaragua. The landscape is hilly, the climate very wet, the vegetation a lush tropical forest. Belt traveled for nearly three weeks by boat and mule to get from the Atlantic coast to Santo Domingo.

His job provided him with enough free time to pursue his natural-history interests and it also gave him a chance to travel within the country. Spending days on end on the back of a mule may not be the most comfortable way to see the country but the pace is ideal. Belt traveled with his net at the ready, and he several times mentions snaring interesting butterflies or beetles as his mule passed them by.

I find his accounts of the ants of Nicaragua the most intriguing parts of the book. He begins by writing of the army ants. My own conception of these insects came straight from Hollywood. I imagined suicidal hordes chasing Fernando Lamas through the jungle devouring every creature that crossed their path, reducing cattle to skeletons in seconds.

The reality is a good deal less lurid. They mainly devour spiders, cockroaches, and various other arthropods, and they aren’t at all suicidal. In fact, they are positively solicitous of each other’s welfare. While observing columns on the move, Belt would cover an ant with a pebble or piece of clay in such a way as to prevent it from escaping without injuring it. When other ants discovered the captivity of their sister, they immediatety crowded around in great numbers and either pushed the pebble away or pulled the captive out from under it.

Many of the insects and spiders that live on the floor of tropical forests have developed means of defense against army ants. Belt saw a green locust remain absolutely immobile while a column of ants marched past it. Some of the ants actually ran over the locust’s legs without realizing they were passing up a meal. Army ants are blind; they find their prey by smell, and they are especially sensitive to carbon dioxide. The locust must have done the insect equivalent of holding its breath until the ants passed.

Belt records another defense, this one by a harvestman or daddy longlegs which he saw “standing in the midst of an army of ants, and with the greatest circumspection and coolness lifting, one after the other, its long legs, which support its body above their reach. Sometimes as many as five of its eight legs would be lifted at once, and whenever an ant approached one of those on which it stood, there was always a clear space within reach to put down another, so as to be able to hold up the threatened one out of danger.”

As a proper Englishman, Belt planted himself a garden as soon as he arrived in Santo Domingo. That brought him into conflict with another of the local ants: the leaf cutters. Leaf-cutting ants are farmers. They cut small pieces of leaves and carry them to their underground nests, where they use them as a mulch for cultivating a fungus, which is what they live on.

Many of the native plants around Santo Domingo have developed chemical defenses the leaf cutters, so their foliage is left alone. But cabbages are totally at the mercy of the leaf cutters, so Belt embarked on a program of chemical warfare of his own, pouring gallons of carbolic acid solution into the ants’ nests and thus murdering large numbers of them, driving the rest away, and making one little corner of the Neotropics safe for the vegetables of Old England.

Belt also discovered the symbiotic relationship between the bull’s-horn acacia and a species of ant. The huge thorns of the acacia are hollow, and the ants live in them. The acacia also provides them with food in the form of a sweet syrup and of tiny fruits that form at the base of young leaves. The ants inspect these regularly and pick them when they are ripe.

In return for food and shelter the ants–which sting quite viciously–protect the plant from both leaf-cutting ants and from herbivorous mammals. In honor of Belt’s discovery of the details of this relationship, the tiny fruits on these and other plants are called Beltian bodies.

Belt closes his book with some reflections on the past and future of the people of Nicaragua that demonstrate perfectly what a bloody, brutal, ghastly set of ideas Darwinism becomes when you attempt to apply it to human societies. He sees natural selection as determining the course of history. If the British–and their American relatives–are on top, it is because selection has made them superior and that superiority is genetic and therefore lasting. In his vision of the future, Mexico will be absorbed into the United States as the first step in the invincible sweep of Anglo-American civilization southward to the Strait of Magellan.

He admits that the Indians of Central and South America once had a civilization, but he suspects they will not create one again. The reason is not the Spanish conquest–although he regards that as the cruel enterprise that it was–but the lack of warfare between tribes. The Indians are in bad shape because their weaklings are reproducing as rapidly as their superior people. The rigorous selection pressure provided by warfare has been removed and the people are suffering for it.

His Darwinian blinders also require that he imagine the civilized Aztecs, Mayans, and Incas to be separate races from their less advanced neighbors. Even good ideas can be carried too far.