David Crystal writes in his book 'How language works':
An interesting problem arises in cases where there is a geographical dialect continuum. There is often a chain of dialects spoken throughout an area. At any point in a chain, speakers of a dialect can understand the speakers of other dialects who live in adjacent area to them; but they find it difficult to understand people who live further along the chain; and they may find the people who live furthest away completely unintelligible. The speakers of the dialects at the two ends of the chain will not understand each other; but they are nonetheless linked by a chain of mutual intelligibility.
This kind of situation is very common. An extensive continuum links all the dialects of the languages known as German, Dutch and Flemish. Speakers in Eastern Switzerland can not understand speakers in Eastern Belgium; but they are linked by a chain of mutually intelligible dialects throughout the Netherlands, Germany and Austria. . There is West Romance continuum which links rural dialects of Portuguese, Spanish, Catalan, French and Italian. We are used to think of these languages as quite different from each other; but his is only because we are usually exposed to their standard varieties, which are not mutually intelligible.
I find a similar language continuum from North to South of Pakistan, especially on the Eastern side of river Indus. Kashmiri Dogri, Hindko, Potohari, Punjabi, Saraiki and Sindhi - all of these languages are connected by a chain of mutual intelligibility.
I think it is probably same with the Chinese dialects.
An interesting problem arises in cases where there is a geographical dialect continuum. There is often a chain of dialects spoken throughout an area. At any point in a chain, speakers of a dialect can understand the speakers of other dialects who live in adjacent area to them; but they find it difficult to understand people who live further along the chain; and they may find the people who live furthest away completely unintelligible. The speakers of the dialects at the two ends of the chain will not understand each other; but they are nonetheless linked by a chain of mutual intelligibility.
This kind of situation is very common. An extensive continuum links all the dialects of the languages known as German, Dutch and Flemish. Speakers in Eastern Switzerland can not understand speakers in Eastern Belgium; but they are linked by a chain of mutually intelligible dialects throughout the Netherlands, Germany and Austria. . There is West Romance continuum which links rural dialects of Portuguese, Spanish, Catalan, French and Italian. We are used to think of these languages as quite different from each other; but his is only because we are usually exposed to their standard varieties, which are not mutually intelligible.
I find a similar language continuum from North to South of Pakistan, especially on the Eastern side of river Indus. Kashmiri Dogri, Hindko, Potohari, Punjabi, Saraiki and Sindhi - all of these languages are connected by a chain of mutual intelligibility.
I think it is probably same with the Chinese dialects.