Lord Visnu maintains and accomplishes all good fortune. Lord Visnu acts through Lord Siva in the creation of the material world. Lord Siva acts on behalf of Lord Visnu. When the Lord says in Bhagavad-gita (14.4) that He is the father of all living entities (aham bija-pradah pita), this refers to actions performed by Lord Visnu through Lord Siva. Lord Visnu is always unattached to material activities, and when material activities are to be performed, Lord Visnu performs them through Lord Siva. Lord Siva is therefore worshiped on the level of Lord Visnu. When Lord Visnu is untouched by the external energy He is Lord Visnu, but when He is in touch with the external energy, He appears in His feature as Lord Siva.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Eighth Canto, Chapter 7 – Text 22