Toynbee points to an evolutionary principle: adaptation can become a disadvantage when it is too closely fitted to a narrow set of conditions. A species may flourish while its specialised environment remains stable, yet possess fewer ways of responding when those conditions change. What once made it successful can therefore restrict its future possibilities.

The wider implication is that adaptability requires more than becoming efficient within present circumstances. Excessive specialisation can reduce flexibility and leave an organism, person or society vulnerable to change. Toynbee uses biology to express a broader warning about development: success within one environment does not necessarily prepare something for the next. The quotation therefore contrasts immediate fitness with long-term resilience. Its underlying principle is that retaining a capacity to adjust may be more important for survival and continued development than becoming perfectly suited to a particular situation.

Full quotation

“Thereby they have set their feet on the path of retrogression. Biologists tell us that animal species which have adapted themselves too nicely to highly specialized environments are at a dead end and have no future in the evolutionary process. That is exactly the fate of the arrested civilizations.”