Srimad Bhagavatam Canto 05, Chapter 14, Text 31

SB 5.14.31

tatrapi niravarodhah svairena viharann ati-krpana-buddhir anyonya-mukha-niriksanadina gramya-karmanaiva vismrta-kalavadhih.
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
In this way the descendants of the monkeys intermingle with each other, and they are generally known as sudras. 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 gramya-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 by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Materialistic people are sometimes called sudras, 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 sudras. Sri Caitanya Mahaprabhu’s mission, this Krsna consciousness movement, is trying to elevate sudras to the brahmana 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 brahmanas. 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 gramya-karmana 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-gita (9.25):
 
yanti deva-vrata devan
 pitrn yanti pitr-vratah
bhutani yanti bhutejya
 yanti mad-yajino ’pi mam
 
“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-guna 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 (yanti mad-yajino ’pi mam). That is the real purpose of human life. This Krsna 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 Srimad-Bhagavatam (1.2.17):
 
srnvatam sva-kathah krsnah
 punya-sravana-kirtanah
hrdy antah-stho hy abhadrani
 vidhunoti su-hrt-satam
 
“Sri Krsna, the Personality of Godhead, who is the Paramatma [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 brahmana, chant the Hare Krsna mantra and read Bhagavad-gita and Srimad-Bhagavatam. In this way one purifies himself of the baser material modes (tamo-guna and rajo-guna) 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 (siddhim paramam gatah).
Srimad Bhagavatam Canto 05, Chapter 14, Text 30
Srimad Bhagavatam Canto 05, Chapter 14, Text 32