"The study of brain and intelligence in non-human vertebrates may eventually throw light on our own thought processes." -- E. M. MacPhail
Why have I chosen to create a website entirely dedicated to the neuroanatomy  and related issues of a number of different species? Why not focus just on humans? What is the value in comparison studies?
A comparative approach allows us to examine the various aspects of diversity in nature. We can examine the relative sizes of brains, the relative sizes of brain structures, and then examine relative behavior of those species. The more information we have, the more questions we can ask.
Total brain size alone may not be a sufficient study since the same size found in different species may be the result of a differential growth of different brain parts. Thus, we will also look at brain composition (brain structures). The characters of structural differentiation can vary in many ways (i.e. in construction, arrangement and connections of the cells, layers, larger units, etc.), while there can be only two variations in total brain size (i.e. smaller or larger).
It is important to realize that an animal's (including humans!) interactions with its environment are controlled largely by the nervous system , and often the size of a certain brain structure (i.e. amount of brain tissue) is highly related with the functional requirements of a species' habits. Thus, can we assume that higher and more complex behavior patterns need larger and more differentiated cerebral structures? We'll think more about that later.
So, we've already started asking questions about behavior. It is obvious that analyzing differences in brain size and structure may ultimately lead to a deeper understanding of the complex relationship between brain and behavior. Let's begin our examination of comparative brain sizes and then move on to comparative brain structures.