The Janus Capital Group and the Henderson Group, a large London-based investment manager, said on Monday that they would merge in an all-stock deal that would create an asset manager with greater global scale and about $320 billion of assets under management.

The deal — creating a company to be called Janus Henderson Global Investors — highlights the extent to which smaller mutual fund companies that focus on actively managed investment offerings are feeling the pressure from the explosive growth of lower cost, passive strategies like exchange traded funds.

The companies said on Monday that combined they would be more stable through market cycles and would have greater geographic reach. They said the merger would also result in annual cost savings of $110 million.

Janus, which lured William H. Gross after he left Pimco two years ago, is strong in North America, with $149 billion in assets under management in the United States. Henderson, which is based in London, is a large investment manager in Europe with about $91 billion in assets under management in Britain and Continental Europe.