CHAPTER VIII. ANALOGIES. ANALOGIES are not proofs, but they facilitate the explanation of abstract ideas and theoretical processes, and with this object we gladly avail ourselves of the developments of the loom and of the steam-engine to illustrate our theory of the evolution of life. The Art of Weaving. The Jacquard loom may be described as a mechanism that, when set in motion and supplied with suitable material, will go on weaving cloth of identically the same texture and pattern, so long as the speed of the machine, the material, and the conditions. remain unchanged. The forces of the loom are of two kinds : one a "general" force-the motive power; and the other-a force specific to every web of different pattern-produces the particular pattern of the web, as the General and Specific life-forces in the germ-cell combine to develop an organism of a specific type. The speed of the loom, the quality of the yarn, and the conditions of manufacture may vary, and any variation will so affect the web that the skilled eye can detect a corresponding difference in the cloth, just as the breeder can recognise different expressions of the same type among the animals he breeds, arising from analogous causes. The art of weaving was developed from the simple to the complex by successive steps or stages; so also the evolution of life. The method of weaving varies, but the underlying principle the interlacing of filaments—is the same throughout; as in life, all growth is by the multiplication of cells. Plants and animals may be classified to illustrate the successive steps in evolution. In like manner products of the loom may be arranged to show clearly the successive stages in the development of weaving, and the several fabrics will have so much in common that the complex might seem to have grown out of the simple,-as in a sense it did. But no intelligent observer would conclude that the brocade of the Duchess was evolved from the mat of the savage by selection, or by environment, or by spontaneous" variation. We know that for every new fabric and every new pattern of the web, the ingenuity of man had to devise a new combination of force, or to prepare a different design. Again, as the fœtus of the mammal exhibits in its growth successive phases in the evolution of life, so successive steps in the evolution of the loom may be recognised in the weaving of complicated fabrics. For example, in manufacturing a damask tablecloth, having a plain border surrounding a twilled square with a pattern of flowers in the centre, the loom weaves first the border on the simple plan of crossed threads the method of the primitive loom; a development is indicated in the weaving of the outer portion of the square-the threads are crossed in a new fashion to throw up a simple pattern-and then a further stage is exhibited in the centrepiece of flowers. To change the centre pattern from flowers to, say, shells, the same combinations that wove the first and second stages of the tablecloth with flowers, weave also the same stages of that with the shell centre. A new combination is necessary only to differentiate the centre patterns; the forces that wove the first cloth are used for the second, so far as they will subserve the new design. And so in the evolution of life, the Specific life-force of the antecessor was utilised in evolving its successor, so far as it would subserve that purpose. The Steam-engine. The steam-engines of the present day, Parson's turbine excepted, have been evolved from the Cornish pumping-engine of a century past. The evolution has been by numberless steps-some insignificant, others might be called gigantic; but, small or large, each was the result of an application of man's intelligence and skill to the improvement of an existing machine; and each successive step may be discovered by a comparative study of the engines successively constructed. Steam-engines vary greatly in character and appearance, but as the naturalist classifies animals, so an expert engineer could classify steam-engines into families, genera, species, and varieties; each clearly defined to the professional eye as as animals, by their classification, to the naturalist. The expert could also explain each successive step in development, and point out the changes in the structure, just as the morphologist can specify the higher, or in some cases the lower specialisation of the different organisms that succeeded each other. Taking a wide, tall, spreading tree as an illustration, the expert might show how the piston and cylinder, like the stem of the tree, have developed in size and character; how the cylinder got its power first from atmospheric pressure only, then from air and steam combined, and later from steam only; while at the same time the stem threw off branches on all sidesrepresented by the condenser, the heater, the fly-wheel, governor, and other improvements, that again on their part developed in character and efficiency. super The expert might then invite attention to the manner in which the changes were effected how the first engines were simplest in construction; how the design of an existing type of engine was utilised, so far as it would serve, for the new; how some organ of the old engine was transformed into something different in the new; and how occasionally a fragment of some member |