So, also, the broad distinction between reproduction by seeds or ova and propagation by buds, though perfect in some of the lowest forms of life, becomes evanescent in others; and even the most absolute law we know in the physiology of genuine reproduction, that of sexual co-operation, has its exceptions in both kingdoms in parthenogenesis, to which in the vegetable kingdom a most curious series of gradations leads.  In plants, likewise, a long and most finely graduated series of transitions leads from bisexual to unisexual blossoms; and so in various other respects.  Everywhere we may perceive that Nature secures her ends, and makes her distinctions on the whole manifest and real, but everywhere without abrupt breaks.  We need not wonder, therefore, that gradations between species and varieties should occur; the more so, since genera, tribes, and other groups into which the naturalist collocates species are far from being always absolutely limited in Nature, though they are necessarily represented to be so in systems.  From the necessity of the case, the classifications of the naturalist abruptly define where Nature more or less blends.  Our systems are nothing, if not definite.  They are intended to express differences, and perhaps some of the coarser gradations.  But this evinces, not their perfection, but their imperfection.  Even the best of them are to the system of Nature what consecutive patches of the seven colors are to the rainbow.

Now the principle of gradation throughout organic Nature may, of course, be interpreted upon other assumptions than those of Darwin’s hypothesis,—­certainly upon quite other than those of materialistic philosophy, with which we ourselves have no sympathy.  Still we conceive it not only possible, but probable, that this gradation, as it has its natural ground, may yet have its scientific explanation.  In any case, there is no need to deny that the general facts correspond well with an hypothesis like Darwin’s, which is built upon fine gradations.

We have contemplated quite long enough the general presumptions in favor of an hypothesis of the derivation of species.  We cannot forget, however, while for the moment we overlook, the formidable difficulties which all hypotheses of this class have to encounter, and the serious implications which they seem to involve.  We feel, moreover, that Darwin’s particular hypothesis is exposed to some special objections.  It requires no small strength of nerve steadily to conceive not only of the variation, but of the formation of the organs of an animal through cumulative variation and natural selection.  Think of such an organ as the eye, that most perfect of optical instruments, as so produced in the lower animals and perfected in the higher!  A friend of ours, who accepts the new doctrine, confesses that for a long while a cold chill came over him whenever he thought of the eye.  He has at length got over that stage of the complaint, and is now in the fever of belief, perchance to be succeeded by the sweating stage, during which sundry peccant humors may be eliminated from the system.