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] It may be objected, and justly, that in the resemblance between crystals and organisms the analogy is purely that of form, and usually confined to one element, whereas between organisms there is resemblance of substance no less than of form, and usually the organisms are alike in several respects. The answer to this objection is, that wherever there is a similarity in the causal conditions (substance and history) there must be a corresponding similarity in the results; if this similarity extends to only a few of the conditions, the analogy will be slight; if to several, deep. But whether slight or deep we are not justified, simply on the ground of resemblance, in assuming, short of evidence, that because they are alike, two organisms are related by descent from a common ancestor. 120. Let us glance at a few illustrations. It has been urged as a serious objection to Mr. Darwin’s hypothesis,[66] that it fails to explain the existence of phosphorescent organs in a few insects; and certainly, when one considers the widely different orders in which these organs appear, and their absence in nearly related forms, it is a difficulty. In noctilucæ, earthworms, molluscs, scolopendra, and fireflies, we may easily suppose the presence of similar organic conditions producing the luminosity; but it requires a strong faith to assign Descent as the cause.[67] We may say the same of the electric organs possessed by seven species of fish, belonging to five widely separated genera. Although each species appears to have a limited geographical range, one or the other is found in almost every part of the globe. These organs occupy different positions, being now on each side of the head, now along the body, and now along the tail; and in different species they are innervated from different sources. Their intimate structure also varies; as appears from the remarkable investigations of Max Schultze.[68] They cannot, therefore, be homologous. How could they have arisen? Not by the slow accumulations of Natural Selection, because, until the organs were fully formed, they could be of no advantage in the struggle; hence the slow growth of the organ must have proceeded without the aid of an advantage in the struggle--in each case from some analogous conditions which produced a differentiation in certain muscles. The fundamental resemblance to muscles was pointed out by Carus long ago. It has been insisted on by Leydig:[69] and Owen says, “The row of compressed cells constituting the electric prism of the Torpedo offers some analogy to the row of microscopic discs of which the elementary muscle fibre appears to consist.”[70] We must not, however, forget that these resemblances are merely such as suggest that the electric organ is a _differentiation_