Why doesn't the full string convert?

Question

I'm trying to remove all whitespace and punctuation marks from a string. But my function only returns the first word of the whole sentence. Where is the problem? Here is my code:

string Convert(string line)
{
    string phrase = "";

    for (int i = 0; i < line.size(); ++i)
    {
        if (line[i] >= 'a' && line[i] <= 'z')
            phrase += line[i];
    }

    return phrase;
}

int main()
{
    string input;
    cin >> input;

    input = Convert(input);
    cout << input;
}