 The crux of this argument is the idea that complex biological systems cannot evolve unless they are organized in a modular way, where modular organization entails that each component of the system (that is, each module) can be selected for change independently of the others. In other words, the evolvability of the system as a whole requires the independent evolvability of its parts. The problem with this assumption is twofold (Woodward &amp; Cowie, 2004). First, not all biological traits are independently modifiable. Having two lungs, for example, is a trait that cannot be changed without changing other traits of an organism, because the genetic and developmental mechanisms underlying lung numerosity causally depend on the genetic and developmental mechanisms underlying bilateral symmetry. Second, there appear to be developmental constraints on neurogenesis which rule out changing the size of one brain area independently of the others. This in turn suggests that natural selection cannot modify cognitive traits in isolation from one another, given that evolving the neural circuitry for one cognitive trait is likely to result in changes to the neural circuitry for other traits. 