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Show 438 MORPHOLOGY. CHAP. XIII. natural selection, during a long-continue.d course of modification should have seized on a certain number of the primordially similar elements, many times repeated, and have adapted them to the most diverse purposes. And as the whole amount of modification will have been effected by slight successive steps, we need no.t wonder at discovering in such parts or organs, a certain degree of fundamental resemblance, retained by the strong principle of inheritance. In the great class of molluscs, though we can homo-logise the parts of one species with those of another and distinct species, we can indicate but few serial homologies; that is, we are seldom e~abled to s~y that one part or organ is homologous with another In the same individual. And we can understand this fact ; for in molluscs, even in the lowest members of the class, we do not find nearly so much indefinite repetition of any one part, as we find in the other great classes of the animal and vegetable kingdoms. Naturalists frequently speak of the skull as formed of metamorphosed vertebrre: the jaws of crabs as meta·morphosed legs ; the stamen? and pi~tils of flowers as metamorphosed leaves ; but It would In these cases probably be more correct, as Professor Huxley has remarked, to speak of both skull and vertebrre, both jaws and legs, &c.,-as having been metamorphosed, not one from .the other but from some common element. Naturalists, howe~er, use uch language only in a me.taphorical sense: they are far from meaning that du~Ing a long course of descent, primordial organs of any lnnd-vertebrre in the one case and legs in the other-have act.ually been modified into skulls or jaws. Yet so stron~ IS the appearance of a modification of this nature havmg ~ccurred, that naturalists can hardly avoid employ~ng language havi.n g thi.s p1a i.n si.g ni'f i ca t'w n. On my view CHAP. XIII. EMBRYOLOGY. 439 these terms. may be used literally; and the wonderful fact of the Jaws, for instance, of a crab retaining numerous characters, which they would probably have retained through inheritance, if they had really been metamorphosed during . a long course of descent from true legs, or from some simple appendage, is explained. Embryology.-It has already been casually remarked that certain organs in the individual, which when mature bec~me widely different and serve for different purposes, a~e .In the ~mbryo. e~actly alike. The embryos, . also, of ~1st~nct animals within the same class are often strikingly s~milar: a better proof of this cannot be given, than a Circumstance mentioned by Agassiz, namely, that having forgotten to ticket the embryo of some vertebrate animal, he cannot now tell whether it be that of a mammal bird, or reptile. The vermiform larvre of moths, flies: beetles, &c., resemble each other much more closely than do the mature insects ; but in the case of larvre t~e e~bryos ~re active, and have been adapted for spe~ Cial hnes of life. A trace of. the law of embryonic res~ mblance, sometimes lasts till a rather late age: thus brrds of the same genus, and of closely allied genera, often resemble each other in their :first and second plumage ; as we see in the spotted feathers in the th~ush group. In ~he cat tribe, most of the species are s~ri~ed ~r spot~ed In lines; and stripes can be plainly distinguished In the whelp of the lion. We occasionally though rarely see something of this kind in plants : thus the embryonic leaves of the ulex or furze, and the fi~s~ leav~s of the phyllodineous acaceas, are pinnate or divided h~e the ordinary leaves of the leguminosre. The pmnts of structure, in which the embryos of widely different animals of the same class resemble each other, often have no direct relation to their condi- |