is possible for a nerve cell to be in one state, whereas some of its close neighbors may be in completely different states. This is in marked contrast to, say, muscle cells. If one muscle cell is involved in a contraction, then nearby cells almost certainly are as well. Neural tissue is quite different. Even the individual states of nerve cells in a network depend critically on the topology of the network itself. Further, assemblages that are actually distinct may have a complex three-dimensional distribution that can be very difficult to untangle. These properties of neural tissue make it exceedingly difficult to "see" the morphology of neural assemblages--with few exceptions, the network topology of virtually our entire brain is currently "invisible." It exists at a scale above the individual cell, but well below that which can be teased apart with any imaging technology currently available. Until recent decades, much of our immune system was similarly "invisible." Evolutionary psychology offers one way around this technological limitation. If researchers had a sound basis for proposing brain functions a priori, they could then seek indirect evidence that brains in fact have these functional properties. Philosophers and scientists had long wondered why living things are made up of an amazing array of beautifully designed mechanisms, an organization which non-living things completely lack. Why is it that entities that reproduce manifest overwhelming evidence of design, but entities that don't reproduce are utterly devoid of the same? As Darwin and Wallace first perceived, the association of reproduction and design is not accidental. Evolution by natural selection is currently accepted as the only process whereby entities can acquire functional properties. Functional organization is the consequence of the reproductive feedback that characterizes natural selection. If a population of reproducing entities (hereafter organisms) varies in some trait, if the variations can be passed on to offspring, and if, as a consequence of possessing a particular variant, an organism produces more offspring on average than organisms that lack that variant over evolutionary time, then the population will come to consist solely of organisms possessing the reproductively efficacious variant trait. In this way, populations of organisms will tend to acquire traits that facilitate reproduction and lose traits that hinder reproduction. We now know that what is passed on to offspring is a large DNA molecule that is further partitioned into numerous sections called genes. Because the structure of this DNA is intimately bound up with the structure of the organism, variations in the DNA are strongly associated with variations in the organism. Changes in DNA are referred to as mutations, and result from environmental hazards such as radiation, toxins, etc. Reproduction is an enormously complex process. At any given moment in the human body, there are thousands of process that, should they fail to complete successfully, would result in death within minutes. For this reason, any given random change in the body is likely to hinder survival and reproduction, not facilitate it. There are far more ways for a mechanism to fail than there are ways to improve it. How many times has