Why do long range axons in mammals travel in white matter tracts? I am curious to know as to why long range, myelinated axons prefer to convene and form white matter tracts, rather than simply reach its target in an arbitrary fashion. Is there some kind of evolutionary advantage to this phenomena? Or is this an ongoing research question?
Wen &amp; Chklovskii (2005) looked at exactly this question through a simulation study. They assumed that the segregation of white and gray matter was the the result of evolutionary pressure to maximize some aspect of connectivity. They tested the idea that simultaneously maximizing interconnectivity (neurons should be able to connect to all other neurons without too many intermediate steps) and minimizing conduction delay (time required for a signal to travel from one neuron to another) would result in the segregation of gray and white matter. 

Through computational simulation, they were able to determine that these simultaneous pressure were sufficient to drive the white/gray matter divide. Part of what they were able to show is that, under some simple geometric assumptions, a mixed (homogeneous) design that had white matter interspersed with gray matter led to much higher conduction delays than a segregated design. The basic reason is that when white matter is mixed in with the gray matter, the short-range connections between nearby neurons in gray matter are longer to account for the interference of the white matter tracts.

Wen, Q., and Chklovskii, D.B. (2005). Segregation of the brain into gray and white matter: A design minimizing conduction delays. PLoS Comput Biol, 1(7): e78. doi:10.1371/journal.pcbi.0010078