Difference Between Convergent and Divergent Evolution. When considering living organisms, evolution can be defined as the development of differentiated organisms from less differentiated pre-existing organisms over the course of time. There are many sources, which provide evidence to the theory of evolution.
You're apparently differentiating between daycare and preschool, right? Private preschools (about $500 per month for three days per week, 9:00a to 1:00p) typically have a minimum age of 30 months (2.5 years) and potty-trained.
And many children are not ready for formal education until four or five years' old. Preschool may be as little a few hours a week to a full-day program, five days a week. As dgraab responds, some preschools require bathroom independence, but others do not. Also, preschool education is not necessarily a requirement.