SB 5.14.31

tatrāpi niravarodhaḥ svaireṇa viharann ati-kṛpaṇa-buddhir anyonya-mukha-nirīkṣaṇādinā grāmya-karmaṇaiva vismṛta-kālāvadhiḥ.
Synonyms: 
tatra api — in that condition (in the society of human beings descended from monkeys); niravarodhaḥ — without hesitation; svaireṇa — independently, without reference to the goal of life; viharan — enjoying like monkeys; ati-kṛpaṇa-buddhiḥ — whose intelligence is dull because he does not properly utilize his assets; anyonya — of one another; mukha-nirīkṣaṇa-ādinā — by seeing the faces (when a man sees the beautiful face of a woman and the woman sees the strong build of the man’s body, they always desire one another); grāmya-karmaṇā — by material activities for sense gratification; eva — only; vismṛta — forgotten; kāla-avadhiḥ — the limited span of life (after which one’s evolution may be degrading or elevating).
Translation: 
In this way the descendants of the monkeys intermingle with each other, and they are generally known as śūdras. Without hesitating, they live and move freely, not knowing the goal of life. They are captivated simply by seeing the faces of one another, which remind them of sense gratification. They are always engaged in material activities, known as grāmya-karma, and they work hard for material benefit. Thus they forget completely that one day their small life spans will be finished and they will be degraded in the evolutionary cycle.
Purport: 

Materialistic people are sometimes called śūdras, or descendants of monkeys, due to their monkeylike intelligence. They do not care to know how the evolutionary process is taking place, nor are they eager to know what will happen after they finish their small human life span. This is the attitude of śūdras. Śrī Caitanya Mahāprabhu’s mission, this Kṛṣṇa consciousness movement, is trying to elevate śūdras to the brāhmaṇa platform so that they will know the real goal of life. Unfortunately, being overly attached to sense gratification, materialists are not serious in helping this movement. Instead, some of them try to suppress it. Thus it is the business of monkeys to disturb the activities of the brāhmaṇas. The descendants of monkeys completely forget that they have to die, and they are very proud of scientific knowledge and the progress of material civilization. The word grāmya-karmaṇā indicates activities meant only for the improvement of bodily comforts. Presently all human society is engaged in improving economic conditions and bodily comforts. People are not interested in knowing what is going to happen after death, nor do they believe in the transmigration of the soul. When one scientifically studies the evolutionary theory, one can understand that human life is a junction where one may take the path of promotion or degradation. As stated in Bhagavad-gītā (9.25):

yānti deva-vratā devān
 pitṝn yānti pitṛ-vratāḥ
bhūtāni yānti bhūtejyā
 yānti mad-yājino ’pi mām

“Those who worship the demigods will take birth among the demigods; those who worship ghosts and spirits will take birth among such beings; those who worship ancestors go to the ancestors; and those who worship Me will live with Me.”

In this life we have to prepare ourselves for promotion to the next life. Those who are in the mode of rajo-guṇa are generally interested in being promoted to the heavenly planets. Some, unknowingly, are degraded to lower animal forms. Those in the mode of goodness can engage in devotional service, and after that they can return home, back to Godhead (yānti mad-yājino ’pi mām). That is the real purpose of human life. This Kṛṣṇa consciousness movement is trying to bring intelligent human beings to the platform of devotional service. Instead of wasting time trying to attain a better position in material life, one should simply endeavor to return home, back to Godhead. Then all problems will be solved. As stated in Śrīmad-Bhāgavatam (1.2.17):

śṛṇvatāṁ sva-kathāḥ kṛṣṇaḥ
 puṇya-śravaṇa-kīrtanaḥ
hṛdy antaḥ-stho hy abhadrāṇi
 vidhunoti su-hṛt-satām

“Śrī Kṛṣṇa, the Personality of Godhead, who is the Paramātmā [Supersoul] in everyone’s heart and the benefactor of the truthful devotee, cleanses the desire for material enjoyment from the heart of the devotee who relishes His messages, which are in themselves virtuous when properly heard and chanted.”

One simply has to follow the regulative principles, act like a brāhmaṇa, chant the Hare Kṛṣṇa mantra and read Bhagavad-gītā and Śrīmad-Bhāgavatam. In this way one purifies himself of the baser material modes (tamo-guṇa and rajo-guṇa) and, becoming freed from the greed of these modes, can attain complete peace of mind. In this way one can understand the Supreme Personality of Godhead and one’s relationship with Him and thus be promoted to the highest perfection (siddhiṁ paramāṁ gatāḥ).