The historian, in fact, is engaged in the study of an unfinished organism, whose development is constantly presenting him with surprises.

It is as if the biologist were suddenly to come upon new and unheard-of species and families which would upset his old classification, or as if the chemist were to find his laws of combination replaced by others which were not only unknown to him, but which were really new and recent in the world.
Other inquirers have the whole of the phenomena with which their science is concerned before them, and they may explore them at their leisure.

The sociologist has only an instalment, most likely a very small instalment, of the phenomena with which his science is concerned before him.

They have not yet happened, are not yet phenomena, and as they do happen and admit of investigation they necessarily lead to constant modification of his views and deductions.
Not only does he acquire new knowledge like other inquirers, but he is constantly having the subject-matter from which he derives his knowledge augmented.

Even in modern times society has thrown out with much suddenness rapid and unexpected developments, of such scope and volume that contemporaries have often lost self-possession at the sight of them, and wondered if social order could survive.