This paper focuses on one of the metaphysical problems for theistic evolution which is the problem of evolutionary transition from one specified substantial form to another. According to the evolutionary account, new substantial forms appear owing to accidental changes in previously existing substances. However, the accidental change may only lead to production of new accidents not entirely new distinct substantial forms. The solutions proposed by modern Thomists head in two directions: One is the reduction of the number of substantial forms (...) ; the other consists of diminishing substantial form altogether. Both proposals stray from classical metaphysics. The evolutionary account of the origin of species ultimately needs to challenge the real existence of species and leads to nominalism. As such it cannot be reconciled with classical metaphysics. (shrink)