Scientists finally reveal what’s inside Charles Darwin’s 200-year-old jars |

For almost two centuries, glass jars holding animals collected by Charles Darwin have remained sealed in museum stores. Any attempt to open them risked evaporation, contamination and irreversible damage to some of the world’s most historically important specimens. As a result, much about their preservation remained uncertain. That has now changed. Scientists from the Natural History Museum, working with researchers at the STFC Central Laser Facility under UK Research and Innovation, have identified the chemical fluids inside Darwin’s jars using a laser-based technique. The breakthrough offers rare insight into early scientific practice and marks a major advance in museum conservation worldwide.

How Charles Darwin collected specimens and stored them in jars

The specimens were gathered during Darwin’s journey aboard the HMS Beagle from 1831 to 1836. During the expedition, Darwin collected mammals, reptiles, fish and invertebrates from South America and the Galápagos Islands. Many later entered the collections of the Natural History Museum in London. These samples remain central to the history of biology.“These jars are more than storage,” said Wren Montgomery, a research technician at the museum. “They are records of how science was done and how practices evolved over time.”Darwin collected animals to answer questions, not to fill shelves. He compared species from different regions to understand variation and adaptation. Mammals and reptiles helped him study bone structure and physical form. Fish revealed how bodies adapt to aquatic life. Invertebrates, including jellyfish, offered insight into simple body plans and development. Together, these specimens formed the evidence base that later shaped his theory of natural selection, years before On the Origin of Species appeared in 1859.

Looking inside sealed jars

Researchers analysed 46 historic specimens using Spatially Offset Raman Spectroscopy, known as SORS. The technique sends laser light through glass and measures how it scatters. That signal reveals the chemical composition of liquids inside sealed containers.Dr Sara Mosca from the STFC Central Laser Facility explained the benefit. “Until now, identifying preservation fluids meant opening jars,” she said. “That risks contamination and chemical change. This method lets us care for fragile specimens while keeping them sealed.”

What chemicals preserved which animals

Clear patterns emerged from the analysis.Mammals and reptiles were usually fixed in formalin soon after collection. This stabilised tissue and stopped decay. They were later transferred to ethanol, which is safer for long-term storage.Fish were also mainly stored in ethanol. Many showed signs of earlier formalin use during initial preservation.Invertebrates, including jellyfish and shrimps, showed the greatest variation. Some were preserved in formalin. Others were kept in buffered water-based solutions. Several samples contained glycerol, likely added to prevent shrinking and collapse in soft-bodied animals.The technique fully identified preservation fluids in about 80 per cent of specimens. Another 15 per cent were partially identified.

Beyond the liquid

The laser scans also revealed whether containers were glass or plastic. This helped track changes in storage over time and past conservation work.“Knowing both the fluid and the container matters,” Montgomery said. “Some combinations age better than others. This gives us early warning before damage becomes visible.”

Why it matters now

More than 100 million fluid-preserved specimens are held in museums worldwide. Many lack detailed records. The ability to analyse them without opening containers could transform how collections are monitored and protected.“This is not just about Darwin,” Mosca said. “It gives museums a way to safeguard irreplaceable material for future research.”

A quiet breakthrough

The study did not uncover new species or hidden biological secrets. Its impact lies in how it protects the past. By confirming exactly how Darwin’s animals were preserved and proving this can be done safely, scientists have set a new standard for conservation.As Montgomery put it, “Understanding what these specimens are stored in helps ensure they survive for the next 200 years.”