. . . no other body can receive a human soul; it cannot enter the body of an animal devoid of reason . . .

. . . Souls who have become unworthy to see God . . . are joined to the bodies of wild beasts . . .

. . . . Creatures often have an evident fitness to the character of the  human beings whose souls are to pass into them . . .

. . . the soul of Thersites assumed an ape, but not the body of the ape . . .

                Phaedrus

. . . the soul descends into savage life, but not into a savage body, for life is conjoined with its proper soul.  And in this place, it is changed into a brutal nature. . . . . For a brutal nature is not a brutal body, but a brutal life. . .

I was once already boy and girl,
Thicket and bird, and mute fish in the waves.
All things doth Nature change, enwrapping souls,
In unfamiliar turns in the flesh,
The worthiest dwellings for the souls of men.

You are not separated from the animals and the rest of the existence by virtue of possession of an eternal inner consciousness.  Such a consciousness is present within all living beings and in all forms.

. . . the learned Tendai School . . . emphasizes that the life in everything  --animals, plants, even drops of water and specks of dust-- is ceaselessly reembodied and will one day reach Buddhahood, the Buddha nature being Potential in all  . . .

. . . I am a great advocate of metempsychosis.  I believe, for a certainty, that they [the lower kingdoms] will ascend to a higher grade of being, and am unable to comprehend how anyone can object to this hypothesis which seems to have the analogy of the whole creation in its favor.

. . . The animals are our younger brothers  and sisters, also on the ladder but a few rungs lower down than we are.  It is an important part of our responsibilities to help them in their ascent, and not to retard their development by cruel exploitation of their helplessness.

. . . the human Ego, if we use it so as to give it only a brutal impression it must fly back to the animal kingdom to be absorbed there instead of being refined and kept on the human plane.  And as all the matter which the human Ego gathered to it retains the stamp or photographic impression of the human being, the matter transmigrates to the lower level when given an animal impress by the Ego . . .

                             The Ocean of Theosophy

. . . The aim for present man is his initiation into complete knowledge, and for the other kindgoms below him that they may be raised up gradually from stage to stage to be in time initiated also . . .

                                 The Heart Doctrine

. . . Nature will not reverse the order of her kingdoms; and while leading the irrational monad of a beast to a higher order into the human form, . . she will not guide that Ego, once it has become a man, even of the lowest king, back into the animal species  --not during that cycle at any rate. . .          --H. P. Blavatsky