Solvable variety of groups

Definition
A solvable variety of groups or soluble variety of groups is a subvariety of the variety of groups (i.e., a collection of groups closed under taking subgroups, quotients, and direct products) satisfying the following equivalent conditions:


 * 1) Every group in the variety is solvable.
 * 2) There exists a nonnegative integer $$c$$ such that every group in the variety is solvable with solvable length at most $$c$$.