The Physical Basis of Mind. George Henry Lewes
the resemblances and differences, there is at present no warrant for assuming that all resemblances and differences are due to this one cause, but, on the contrary, we are justified in assuming a deeper principle which may be thus formulated: All the complex organisms are evolved from organisms less complex, as these were evolved from simpler forms; the link which unites all organisms is not always the common bond of heritage, but the uniformity of organized substance acting under similar conditions.
It is therefore consistent with the hypothesis of Evolution to admit a variety of origins or starting-points, though not consistent to admit the sudden appearance of complex Types, such as is implied in the hypothesis of specific creations.
119 a. The analogies of organic forms and functions demand a more exhaustive scrutiny than has yet been given them. Why is it that vessels, nerves, and bones ramify like branches, and why do these branches take on the aspect of many crystalline forms? Why is it that cavities are constantly prolonged in ducts, e.g. the mouth succeeded by the œsophagus, the stomach by the intestines, the bladder by the urethra, the heart by the aorta, the ovary by the oviduct, and so on? Why are there never more than four limbs attached to a vertebral column, and these always attached to particular vertebræ? Why is there a tendency in certain tissues to form tubes, and in these tubes commonly to assume a muscular coat?64 To some of these queries an answer might be suggested which would bring them under known physical laws. I merely notice them here for the sake of emphasizing the fact that such analogies lie deeply imbedded in the laws of evolution, and that what has been metaphorically called organic crystallization will account for many similarities in form, without forcing us to have recourse to kinship. To take a very simple case. No one will maintain that the crystalline forms of snow have any kinship with the plants which they often resemble. Mr. Spencer has noticed the development of a wing-bearing branch from a wing of the Ptilota plumosa, when its nutrition is in excess. “This form, so strikingly like that of the feathery crystallizations of many inorganic substances, proves to us that in such crystallizations the simplicity or complexity of structure at any place depends on the quantity of matter that has to be polarized at that place in a given time. How the element of time modifies the result, is shown by the familiar fact that crystals rapidly formed are small, and that they become larger when they are formed more slowly.”65
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