As animals have evolved large body size, they have also evolved adaptations to improve exchange of energy and materials with the environment. for example, in many larger organisms, evolution has favored lungs and a digestive tract with

In many larger organisms, evolution has favored lungs and a digestive tract with more branching or folds. Evolution is the change in the characteristics of a species over several generations and relies on the process of natural selection. The heritable characteristics of biological populations change over successive generations with a diversity at every level of biological organization, including the levels of species, individuals organisms, and molecules. 

When you are making a guess you are forming a hypothesis