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Śrīmad-Bhāgavatam
[Canto 6, Ch. 10]
animal, tree or plant, all living entities are sons of the Supreme Personality of Godhead. Lord Kṛṣṇa says in  Bhagavad-gītā  (14.4):
sarva-yoniṣu kaunteya mūrtayaḥ sambhavanti yāḥ tāsāṁ brahma mahad yonir ahaṁ bīja-pradaḥ pitā
“It should be understood that all species of life, O son of Kuntī, are made possible by birth in this material nature, and that I am the seed-giving father.” The different forms of these living entities are only their external dresses. Every living being is actually a spirit soul, a part and parcel of God. Therefore one should not favor only one kind of living being. A Vaiṣṇava sees all living entities as part and parcel of God. As the Lord says in  Bhagavad-gītā  (5.18 and 18.54):
vidyā-vinaya-sampanne brāhmaṇe gavi hastini śuni caiva śvapāke ca paṇḍitāḥ sama-darśinaḥ
“The humble sage, by virtue of true knowledge, sees with equal vision a learned and gentle  brāhmaṇa , a cow, an elephant, a dog and a dog-eater [outcaste].”
brahma-bhūtaḥ prasannātmā na śocati na kāṅkṣati samaḥ sarveṣu bhūteṣu mad-bhaktiṁ labhate parām
“One who is transcendentally situated at once realizes the Supreme Brahman and becomes fully joyful. He never laments nor desires to have anything; he is equally disposed to every living entity. In that state he attains pure devotional service unto Me.” A Vaiṣṇava, therefore, is truly a perfect person because he laments to see others unhappy and feels joy at seeing others happy. A Vaiṣṇava is  para-duḥkha-duḥkhī ; he is always unhappy to see the conditioned souls in an unhappy state of materialism.