The present is really a continuation of the preceding chapter. If it is true- that instincts are modified nowadays by the hereditary adding of new habits, it must be admitted that they were modified in this way formerly during the long periods in which the history of each species was being unfolded. But what has this evolution been? The problem appears unsolvable, for each species has its history, and, in the domain of instincts still more than in that of structure, the past lends itself but little to observation. It is not impossible, however, to arrive at an approximate solution. Each group, as a matter of fact, however big it may be, has followed an evolution of its own, and in each group all the species are not actually in the same stage of this evolution. Some have rested at certain stages of the journey; others have advanced more or less. So by comparing these species, the ones with the others, it is possible approximately to reconstitute the evolutionary phenomena which have been produced in a group in the course of its history.
This is what we are going to try to do for the predatory Hymenoptera of the family Pompilid.