Cambios in Spanish
When we compare a number of Spanish words with other Romance languages or Latin, we see the f has often been changed into an h. ( and dropped in the pronunciation) . F.i. facere -> hacer, ficus, fig -> higo, fil -> hilo, filius, filia -> hijo, hija, forno -> horno, farina -> harina , ferrum -> hierro, fungus -> hongo etc.
Is there a kind of system involved, and if so, where can I find some information on this phenomenon?
3 Answers
It is very common in Latin words that start with f unless it is followed by an R (and sometimes, an L), and it is recorded between the 10th and the 15th centuries.
Consonant and vowel changes happen all the time in all languages in the world, if you wait long enough. Examples of the f -> h in other languages are: the word "funhar" in Tahaggart changed to "hunhar" in Ghadames (both Tuareg dialects), or the old Tahitian word "pufa" changing into "puha". Apparently, Hausa speakers have turned the English word "photo" into "hoto" in just under a century. (Examples from Paul Foulkes)
What exactly do you want to know?
P. S. For more information, check any book of Spanish historical phonetics.
Thanks. As as matter of fact, I know about the first and second Germanic sound shift, which is limited to the Germanic languages and was first described by Jacob Grimm. I started studying Spanish a while ago and was struck by a phenomenon that looks a bit familiar. I could do with some advice about Spanish historical phonetics. I suppose the practical use is limited, but it' s just fun. At least, for some people. 
