Thank you both Dr Shabbir and Sister UmeAimon for your help.
I was in no doubt about the Prophet's true character (sorry if I gave the wrong impression), but was curious about the historical origin of the custom that is prevalent today. Such information is of course useful in rebutting the false claims about the Prophet. What you have said Dr. Shabbir about the fact of daughter infantcide being dealt with in the Quran is a valid point for proving that the custom has its origins in later times. I'd never considered it before. Many thanks.