The book is a travelogue and biological record, but also reveals the two men's philosophies: it dwells on the place of humans in the environment, the interconnection between single organisms and the larger ecosystem, and the themes of leaving and returning home. A number of ecological concerns, rare in 1940, are voiced, such as an imagined but horrific vision of the long term damage that the Japanese bottom fishing trawlers are doing to the sea bed.
The book is a travelogue and biological account, but also exposes the two men's philosophies: it focuses on humanity's position in the environment, the connection between individual organisms and the larger ecosystem, and the themes of departure and return home. Several ecological concerns, uncommon at that time, are expressed, such as an envisioned but terrifying vision of the long-term harm caused by Japanese bottom-fishing trawlers to the sea floor.