The Soul Can Degrade into Lower Species.
The following is a verse from the Bhagavad-gita As It Is by His Divine Grace A.C.Bhaktivedanta Swami Prabhupada where Lord Krsna explains about the various bodies one attains if he is predominantly surcharged with the modes of passion and ignorance.

rajasi pralayam gatva
karma-sangisu jayate
tatha pralinas tamasi
mudha-yonisu jayate
When one dies in the mode of passion, he takes birth among those engaged in fruitive activities; and when he dies in the mode of ignorance, he takes birth in the animal kingdom.