In its earliest meaning in private cult, the genius of the Roman housefather and the iuno, or juno, of the housemother were worshiped. These certainly were not the souls of the married pair, as is clear both from their names and from the fact that in no early document is there mention of the genius or iuno of a dead person. The genius and iuno were probably the male and female forms of the family’s, or clan’s, power of continuing itself by reproduction, which were in the keeping of the heads of the family for the time being and passed at death to their successors. In this as in all forms of his cult, the genius was often conceived as appearing in the form of a snake, although he is also shown in art as a young man, generally engaged in sacrificing. At every wedding a bed, the lectus genialis, was made for the genius and iuno of the husband and wife, and its presence in the house was a sign of matrimony.

Owing to the rise of individualism and also to the prevalence of Greek ideas concerning a guardian spirit, or daimon, the genius lost its original meaning and came to be a sort of personification of the individual’s natural desires and appetites. Hence the phrases indulgere genio, genium defrudare, signifying, respectively, to lead a pleasurable life, and to lead a stingy life. The development, however, did not stop here. The genius came to be thought of as a sort of guardian angel, a higher self; and, as the Greek daimon was sometimes rationalized into the individual’s character or temper, so also the poet Horace half-seriously said that only the genius knows what makes one person so different from another, adding that he is a god who is born and dies with each one of us. This individual genius was worshipped by each individual, especially on his birthday. A few inscriptions even mention the genius of a dead person, as Christian epitaphs sometimes speak of his angel.

To show reverence for the genius of another or to swear by it was a mark of deep respect; hence, it is not unnatural that the genius of Augustus and of his successors formed objects of popular cult. Thus, to worship the genius Augusti avoided affronting the feeling against worshipping any living emperor, which remained fairly strong in Italy; for, of course, all genii were divine and might properly be worshipped.

As with the Greek daimones, there was a vast variety of genii, or guardian spirits—those of places, genius loci, including buildings (genius balneorum, etc.), and of corporations of all sorts, from the state (genius populi Romani) to small bodies of troops, guilds of tradesmen, and so forth. A very curious development is that one sometimes heard of the genius of a god, even of Jupiter, or of the iuno of a goddess.

Thank you Encyclopaedia Britannica.