Gita does not talk of Rebirth into Lower species

Gita does not talk of rebirth of man into lower species or beings of lower than human consciousness. Gita is talking of rebirth of man into lower or higher gradation of human consciousness depending on his acts in this life.  

(14/15) मूढयोनिषुq [mūḍhayoniṣu] doesn't mean beings lower than human beings, it means ignorant men. Gita says that a Tamasic man is born among ignorant men (men of Tamasic consciousness). 

(14/18) अधो गच्छन्ति [adho gacchanti] doesn't mean transmigration to beings lower than human beings. It means that a Tamasic man is born among lower men, men of Tamasic consciousness.

(16/19) आसुरिषु योनिषु doesn't mean transmigration to the world of Asura. It means that a man of Asuric nature is born into Asuric consciousness.

(16/20) अधमाम् गतिम् doesn't mean transmigration to lower beings. It means lowest of Asuric consciousness.

              रजसि प्रलयं गत्वा कर्मसङ्गिषु जायते ।

तथा प्रलीनस्तमसि मूढयोनिषु जायते ।। 14-15।।

14/15. Meeting with dissolution when Rajas prevails, he is born among those attached to action; and if dissolved when Tamas prevails, he is born in the wombs of beings involved in nescience. (*)

ऊर्ध्वं गच्छन्ति सत्त्वस्था मध्ये तिष्ठन्ति राजसाः ।

जघन्यगुणवृत्तिस्था अधो गच्छन्ति तामसाः ।। 14-18।।

14/18. Those who dwell in Sattwa rise upwards; those in Rajas remain in the middle; and those who are in Tamas, they, dwelling in the tendencies of the lowest Guna, go downwards. (*)

तानहं द्विषतः क्रूरान्संसारेषु नराधमान् ।

क्षिपाम्यजस्रमशुभानासुरीष्वेव योनिषु ।। 14-18।।

16/19. These evil and cruel haters, the vilest among men, I cast down constantly into Asuric wombs in the world. (*)

आसुरीं योनिमापन्ना मूढा जन्मनि जन्मनि ।

मामप्राप्यैव कौन्तेय ततो यान्त्यधमां गतिम् ।। 16.20।।

16/20. Cast into Asuric wombs birth after birth, these deluded ones, O Kaunteya, attain not to Me and sink down into the lowest status of soul-nature. (*)

Sri Aurobindo on Rebirth into lower beings:

“These words [mūḍhayoniṣu or adho gacchanti] do not necessarily refer to the animal birth, but it is true that there has been a general belief of that kind not only in India but wherever “transmigration” or “metempsychosis” was believed in. Shakespeare is referring to Pythagoras's belief in transmigration when he speaks of the passage of somebody's grandmother into an animal. But the soul, the psychic being, once having reached the human consciousness cannot go back to the inferior animal consciousness any more than it can go back into a tree or an ephemeral insect. What is true is that some part of the vital energy or the formed instrumental consciousness or nature can and very frequently does so, if it is strongly attached to anything in the earth life. ---” - The letters on Yoga-1,  no. 699.

(*) Translation of the Shalokas is from “The Bhagwat Gita” by P. P. Khetan