We find by other living creatures that G-d ingrained the means with which they sustain themselves into their very nature. For example, He ingrained in the nature of a cat to prey on mice, in the nature of the hawk to prey on those birds that are fitting for it, and in the nature of deer to prey on snakes. Similarly, there are certain birds that will only prey on fish. In a similar fashion, G-d ingrained in the nature of each and every living species a predisposition and desire for one of the species of plants or living creatures to serve as its source of food. Even the structure of the body and limbs of each species is specifically to make them fit for the consumption of the food that it is in their nature to eat, as is evidenced by the long mouth and long legs of the bird that prey on fish, by the teeth and sharp nails of the lion, and by the horns of the bull and the ram. However, for those animals whose food is from vegetation, G-d did not give them body parts to use as instruments with which to hunt and prey.
In a similar manner, you will find that the various character traits of people and the structure of their bodies are each suited for a specific type of business or work. A person who finds that his character traits are suited for a certain work, and that he also desires the work by nature, and that his body is suited for it, and that he is able to tolerate the exertion required for it, should pursue it and make it his means of obtaining his livelihood.
From Chovot Halevavot
