As there are many theorists who feel that if individuals don't progress via a series of changes, in a certain order, than their development will be influenced.
Take Piaget's Theory of Cognitive Development for illustration. There are four phases of cognitive growth: sensorimotor, preoperational, concrete operational and then formal operational thinking. If certain concepts are not mastered (that is, egocentrism and conservation in the preoperational phase) then advancement to the next phase will be compromised.
The similar can be state for socio emotional development. Erikson's theory asserts that individuals are experiencing sets of crises all through their development and successful resolutions of these crises will influence movement through his remaining phases. For illustration, the central crisis is early adulthood is intimacy versus isolation. In order to successfully navigate this phase, a young adult should find a partner to share intimacy with or be faced with being isolated from society. Successful resolution of this phase is based on resolution of the identity versus role confusion crises which was confronted in adolescence. Individuals who have not established an identity in adolescence would be not able to establish intimacy in young adulthood.
These are just two illustrations however can you think of other theories and illustrations illustrated in the readings which support this "phase" idea? And do you agree or disagree with the concept of phases?