While there is no doubt that all divisions of the brain have co-evolved over time, we choose to study the neocortex – the hallmark of mammalian brain evolution – due to the abundance of observed neocortical phenotypic variability across the nearly 7000 extant mammalian species (Mountcastle 1997, Kaas and Balaram 2015, Burgin et al., 2018). At the systems-level, this variability lies in the parcellation of the neocortex and thus in the information that is processed within and across cortical minicolumns (the fundamental computational units of the six-layered neocortex) (Bastos et al., 2018, Garey 1999). One way to parcellate the cortex is by cortical field, each with unique connectivity and function (Krubitzer 2007). Composed of minicolumns that share a common general function, cortical fields are classically defined by electrophysiological properties, connections, and cytoarchitecture. Species display variability in neocortical and cortical field organization, specifically in 1) the size of the cortical sheet, 2) how sensory domains are allocated, 3) the relative size of cortical fields, 4) the number of cortical fields, 5) the magnification of behaviorally relevant body parts within cortical fields, and 6) the connections within and across cortical fields (Krubitzer and Prescott 2018). Evolutionary tinkering with peripheral morphology, subcortical connections, and the neocortical developmental program has produced these systems-level changes to the neocortex, fundamentally altering the function of the neocortex and thus the behaviors animals can engage in (Finlay and Uchiyama 2015, Florio et al., 2015). As behavior is the target of selection, only cortical properties and mechanisms that support/co-vary with a certain behavior are selected for (such as network connectivity, neural response properties, or gene expression).Over short and long timescales, (within and across lifetimes), an individual’s behavior can change as the physical and social environments change. Discussed in detail below, the neocortex has a remarkable ability to adapt to changes on long and short timescales. While neocortical plasticity serves as a buffer against environmental variation, some properties of the cortex are resistant to change (such as the existence of six-layers or the general location of cortical areas), which limits the degree of change that can occur within the lifetime of individual, and places constraints on potential species-level differences (Allman 1999). Using the comparative approach, the Krubitzer lab has studied the differences between within-lifetime and evolutionary changes that are possible to the neocortex. The following chapters extend this body of work, by providing answers to three specific questions. First, how can experimental manipulations in the laboratory inform us of the types of changes that occur over evolutionary time? The first chapter answers this by providing a review of the types of changes to the neocortex that occur over short and long timescales, and how experimental models of sensory loss provide information on how the developmental program can be altered over the course of evolutionary time to produce phenotypic variation. Second, how does early sensory loss alter the neocortical developmental program, and are there behavioral correlates to these changes in adulthood? To answer this question, chapters 2 and 3 present data on the early loss of vision in short-tailed opossums (Monodelphis domestica), extending previous findings from the Krubitzer laboratory on cross-modal plasticity. I first provide a behavioral correlate to the previously observed cross-modal changes, showing that blind animals are better at performing sensorimotor tasks when those tasks rely on the spared senses. Second, I provide a drafted manuscript on how the developmental program is altered by the early loss of vision. Specifically, I show that the loss of vision causes transient corticocortical projections to stabilize in blind animals, allowing for cross-modal plasticity. Third, how does the developmental program vary between species in early postnatal development? In this line of research, and the last chapter presented in this dissertation, I present a new technique for cross-species comparisons of cortex-wide in-situ hybridization. I use this technique to demonstrate some similarities and differences in the cortical developmental program between two rodent species (mice and voles). This last work is currently in revision, and we are adding additional developmental timepoints to address differences in trajectory (versus a single day in development). Taken together, the work presented in this dissertation makes two important contributions to the field of neuroscience. First, we show how the age of onset of sensory loss impacts cortical plasticity, and that the observed effects are due to a lack of axon pruning in development and not a growth of new connections. Second, cortex-wide gene expression patterns have never been quantitatively compared between species. Altogether, these chapters provide data on the limits of neocortical plasticity within and across species. Importantly, learning how and where evolution has tinkered with the neocortical developmental program over all time to produce species variation informs us of the rules of cortical and brain evolution. I hope that other scientists not only enjoy reading these papers, but also apply these new techniques in their research to answer longstanding questions of cortical development and evolution.