The far-reaching significance of these facts in connection with human health and progress become apparent when considered in more detail. Observers throughout the ages have gradually noted, and subsequently turned to practical account in garden, meadow, and farmyard, certain characteristics common to all known forms of vegetable and animal life. By due consideration of these it was found possible to improve breeds, to strengthen and lengthen life, to avert disease, and generally to enhance economic value. It may now appear simple enough, to extend and apply these observations to the betterment of human life; but many generations of human beings slipped away before the facts, dimly discerned by Aristotle and Lucretius, by Buffon and Lamarck, were clearly focussed by Darwin,[91] Wallace, Spencer, and Huxley, through whose skill and labours the continuity of the web of life was first displayed to the world at large. The design may here be almost elementary in its delicate simplicity; there its subtle intricacies well-nigh baffle description. The variety of pattern is marvellous indeed, as Nature weaves with ceaseless industry the woof of progressive development. But the warp of this wondrous web is nevertheless continuous throughout its length, uniting the whole into one vast fabric. This basic unity of all manifestations of life has been further substantiated by another group of scientists--Schleiden, Schwann, Kölliker, and Virchow, for instance--who gradually and conclusively proved the identity, in their simplest form, of those living bricks (_i.e._ microscopic particles of protoplasm) from which the whole vast edifice of life is constructed. The capacity they possess for differentiation in their functions and in modes of combination long masked recognition of the fact that each commonwealth of cells, whether plant or animal, is developed in the first instance in orderly progression from a similar minute speck of protoplasm, acceptation of which has sufficed to bring about a complete revolution in the scientific world. To these discoveries were shortly added Pasteur's conception of the nature and causation of infective diseases; a knowledge which brought with it a great accession of power over hitherto mysterious and uncontrollable conditions. And finally, man's eyes have been opened to the comprehension of Nature's means of self-defence against the micro-organisms of disease. Thus, while humanity is by these means armed with most potent weapons against the inroads of infection, decay, and death, the light thrown upon the mystery of the origin of each individual life has shown man his true place in the kingdom of nature. The application of these great discoveries, together with increased opportunity for and accuracy in their utilisation, constitute the basis of the modern methods of hygiene.