In the later Vedic period, agriculture had become an important activity of the people. Changes in the material life naturally resulted in a change in their attitude towards gods and goddesses too. Continuous interactions with the local non-Aryan population also contributed to these changes.
Thus, Vishnu and Rudra which were smaller deities in the Rigveda became extremely important. However, we do not have any reference to different incarnations or avataras of Vishnu, we are so familiar with, in any of the Later Vedic texts.
Another important feature was the increase in the frequency and number of the yajna which generally ended with the sacrifices of a large number of animals. This was probably the result of the growing importance of a class of brahmanas and their efforts to maintain their supremacy in the changing society. These yajnas brought to them a large amount of wealth in form of dana and dakshina.
Some of the important yajnas were - ashvamedha, vajapeya, rajasuya, etc. In these yajnas which continued for many days a large part of gifts went to the brahmanas. The purpose of these yajnas was twofold. Firstly, it established the authority of the chiefs over the people, and secondly, it reinforced the territorial aspect of the polity since people from all over the kingdom were invited to these sacrifices.
People began to oppose these sacrifices during the later Vedic period itself. A large number of cattle and other animals which were sacrificed at the end of each yajna must have hampered the growth of economy. Therefore, a path of good conduct and self-sacrifice was recommended for happiness and welfare in the last sections of the Vedas, called the Upanishads.
The Upnishads contain two basic principles of Indian philosophy viz., karma and the transmigration of soul, i.e., rebirth based on past deeds. According to these texts real happiness lies in getting moksha i.e. freedom from this cycle of birth and re-birth.