Salmon's Alleged Circle

In the logician Wesley C. Salmon's book Logic (2. edn., 1973), he has a chapter entitled 'Logic and Language' where in the paragraph 'Definitions' he describes language as a conventional entity exemplified with the word cat. However, it appears, surprisingly, that a logical circle intrudes his exhibition. Salmon writes:

»The meaning of a word is not a natural attribute which man discovers; meaning is given to a word by people who agree to let it have that meaning. For example, there is no intrinsic characteristic of the word “cat” which makes it refer to feline animals; it does so because English-speaking people have adopted a convention to that effect. This is not intended to suggest that people once sat down at a conference table and formally decided the meanings of words. For the most part, these conventions, like many other conventions, have grown gradually and informally over a long period of time. As language continuous to grow and develop, these conventions are still subject to change. The important fact is that other conventions could have been adopted without being false or incorrect. […] A word has meaning if there is a convention establishing its meaning.«

But if this is the case, the only evidence of anyone having 'adopted the convention' is that cat now means what it does. The explanation appears to be circular: that is the concealed argument is: 'They must have adopted the convention because otherwise I cannot explain the regularity with which they agree about the meaning of cat.'  

To be sure: Is it the case that Salmon has made a logical circle here?

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