THE contrast between works on biology which were written before the appearance of Darwin's Origin of Species and those which have appeared since that great work is most striking. There can be no doubt that biologists have got hold of a most fruitful hypothesis, and the conceptions introduced by Darwin have shed a great light on the sciences which deal with life. Things which seemed to be far apart and isolated from one another have suddenly been seen to be closely connected, and structures and organisms are seen to be related to one another, and to be parts of an intelligible whole. The full and adequate appreciation of the worth of the facts and of the laws can be grasped completely only by those who are specially qualified; but one who is not a specialist may apprehend the breadth and grandeur of the conception which enables him to think of all life as a unity and to trace the innumerable living forms to slow variation from a single stock. This conception leaves the mystery of life where it found it: origins lie beyond the action of this conception. Science tells us that life comes from life, and it is powerless to explain the origin of life. Let life be given, and science says it can trace its path of progress, and understand some of the laws which have guided its development.