Not indeed till the late Renaissance can we say that the idea of Progress became in any real sense incorporated with the common thought of Western civilization. From then to the present it has suffered many vicissitudes. Starting in the XVIIth century as little more than a consciousness of the superiority of the present over the past, in the XVIIIth it changed to a dogma, its adherents claiming that there existed a “Law of Progress” leading inevitably to the perfectioning of humanity. In the XIXth century the dogma was questioned, and thinkers began to put it to the test--the test of comparing theory with historical fact. A new lease of life, however, was given to the idea of a law of progress by the evolution theory; but finally, of late years, there has been a marked reaction, leading not only to a denial of any such inevitable law, but often to a questioning of the very existence of Progress in any shape or form. It is the business of the philosopher and of the biologist to see whether this scepticism be justified, and to find out by a more scientific approach how much of the doctrine of Progress is valid. To the layman it would seem inevitable, once the validity of the evolution theory was granted, to concede the fact of Progress in some form or another. If we accept the doctrine of evolution, we are bound to believe that man has arisen from mammals, terrestrial from aquatic forms, vertebrates from invertebrates, multicellular from unicellular, and in general the larger and the more complex from the smaller and simpler. To the average man it will appear indisputable that a man is _higher_ than a worm or a polyp, an insect _higher_ than a protozoan, even if he cannot exactly define in what resides this highness or lowness of organic types. It is, curiously enough, among the professional biologists that objectors to the notion of biological progress and to its corollary, the distinction of higher and lower forms of life, have chiefly been found. I say curiously enough, and yet to a dispassionate observer it is perhaps not so curious, but only one further instance of that common human failing, the inability to see woods because of the trees that compose them. That is as it may be. Our best course will be to start by examining some of the chief objections to the idea of biological progress, in order to see if they involve errors of thought which we may then avoid. The most widespread of all the objections raised may, I think, be fairly put as follows: “The fundamental attribute of living beings is adaptation to environment. A man is not better adapted to his environment than the flea which lives upon him as a parasite, or than the bacillus which kills him, nor is a bird better adapted to air than a jelly-fish to water; therefore we have no right to speak of one as higher than the other, or to regard the transition from one type to another as involving progress.”