I have the 1977 reprint of the 1974 edition.

In it there is the following:

PREFACE TO SECOND EDITION

...The exhaustion a few years ago of the first edition of this work, and the calls of a worldwide readership for a second edition, led the author to undertake a thorough revision of the first. The general format remains much as it was, with no alterations made just for the sake of change but only where further thought and information seemed to call for it. The assembly of the colour plates in a section by themselves is intended to make reference from the text easier, since the former scattering of the plates made the required one more troublesome to find, for they rarely coincided with the related text...