Tusi has about 150 works, of which 25 are in Persian and the remaining are in Arabic, and there is one treatise in Persian, Arabic and Turkish.
During his stay in Nishapur, Tusi established a reputation as an exceptional scholar. Tusi’s prose writings represent one of the largest collections by a single Islamic author. Writing in both Arabic and Persian, Nasir al-Din Tusi dealt with both religious (“Islamic”) topics and non-religious or secular subjects (“the ancient sciences”). His works include the definitive Arabic versions of the works of Euclid, Archimedes, Ptolemy, Autolycus, and Theodosius of Bithynia.
In his Akhlaq-i-Nasri, Tusi put forward a basic theory of the evolution of species almost 600 years before Charles Darwin was born. He begins his theory of evolution with the universe once consisting of equal and similar elements. According to Tusi, internal contradictions began appearing, and as a result, some substances began developing faster and differently from other substances. He then explains how the elements evolved into minerals, then plants, then animals, and then humans. Tusi then goes on to explain how hereditary variability was an important factor for biological evolution of living things:
The organisms that can gain the new features faster are more variable. As a result, they gain advantages over other creatures. […] The bodies are changing as a result of the internal and external interactions.
Tusi discusses how organisms are able to adapt to their environments:
Look at the world of animals and birds. They have all that is necessary for defense, protection and daily life, including strengths, courage and appropriate tools [organs] […] Some of these organs are real weapons, […] For example, horns-spear, teeth and claws-knife and needle, feet and hoofs-cudgel. The thorns and needles of some animals are similar to arrows. […] Animals that have no other means of defense (as the gazelle and fox) protect themselves with the help of flight and cunning. […] Some of them, for example, bees, ants and some bird species, have united in communities in order to protect themselves and help each other.