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 record, but it also delves into the philosophical views of the two men: it examines humanity's place in the environment, the interdependence between individual organisms and their surroundings, and the themes of departure and return. The book addresses several ecological concerns that were rare at the time, such as the harm caused by Japanese bottom-fishing trawlers to the ocean floor.