is an eigenvector of the covariance matrix of shape variables. The first axis accounts for maximum variation in the sample, with further axes representing further ways in which the samples vary. The pattern of clustering of samples in this morphospace represents similarities and differences in shapes, which can reflect phylogenetic relationships. As well as exploring patterns of variation, Multivariate statistical methods can be used to test statistical hypotheses about factors that affect shape and to visualize their effects, although PCA is not needed for this purpose unless the method requires inverting the variance-covariance matrix. Landmark data allow the difference between population means, or the deviation an individual from its population mean, to be visualized in at least two ways. One depicts vectors at landmarks that show the magnitude and direction in which that landmark is displaced relative to the others. The second depicts the difference via the thin plate splines, an interpolation function that models change between landmarks from the data of changes in coordinates of landmarks. This function produces what look like deformed grids; where regions that relatively elongated, the grid will look stretched and where those regions are relatively shortened, the grid will look compressed. Ecology and evolutionary biology D'Arcy Thompson in 1917 suggested that shapes in many different species could also be related in this way. In the case of shells and horns he gave a fairly precise analysis… But he also drew various pictures of fishes and skulls, and argued that they were related by deformations of coordinates. Shape analysis is widely used in ecology and evolutionary biology to study plasticity, evolutionary changes in shape and in evolutionary developmental biology to study the evolution of the ontogeny of shape, as well as the developmental origins of developmental stability, canalization and modularity. Many other applications of shape analysis in ecology and evolutionary biology can be found in the introductory text: Neuroimaging In neuroimaging, the most common variants are voxel-based morphometry, deformation-based morphometry and surface-based morphometry of the brain. Bone histomorphometry Histomorphometry of bone involves obtaining a bone biopsy specimen and processing of bone specimens in the laboratory, obtaining estimates of the proportional volumes and surfaces occupied by different components of bone. First the bone is broken down by baths in highly concentrated ethanol and acetone. The bone is then embedded and stained so that it can be visualized/analyzed under a microscope. Obtaining a bone biopsy is accomplished