Modification by selection of the forms of I have above remarked that the feline1 form is quite of secondary importance to the embryo and to the kitten. Of course, during any great and prolonged change of structure in the mature animal, it might, and often would be, indispensable that the form of the embryo should be changed; and this could be effected, owing to the hereditary tendency at corresponding ages, by selection, equally well as in mature age: thus if the embryo tended to become, or to remain, either over its whole body or in certain parts, too bulky, the female parent would die or suffer more during parturition; and as in the case of the calves with large hinder quarters2, the peculiarity must be either eliminated or the species become extinct. Where an embryonic form has to seek its own food, its structure and adaptation is just as important to the species as that of the full-grown animal; and as we have seen that a peculiarity appearing in a caterpillar (or in a child, as shown by the hereditariness of peculiarities in the milkteeth) reappears in its offspring, so we can at once see that our common principle of the selection of slight accidental variations would modify and adapt a caterpillar to a new or changing condition, precisely as in the full-grown butterfly. Hence probably it is that caterpillars of different species of the Lepidoptera differ more than those embryos, at a corresponding early period of life, do which remain inactive in the womb of their parents. The parent during successive ages continuing to be adapted by selection for some one object, and the larva for quite another one, we need not wonder at 1 See p. 42, where the same illustration is used. the difference becoming wonderfully great between them; even as great as that between the fixed rock-barnacle and its free, crab-like offspring, which is furnished with eyes and well-articulated, locomotive limbs1. Importance of embryology in classification. We are now prepared to perceive why the study of embryonic forms is of such acknowledged importance in classification2. For we have seen that a variation, supervening at any time, may aid in the modification and adaptation of the full-grown being; but for the modification of the embryo, only the variations which supervene at a very early period can be seized on and perpetuated by selection : hence there will be less power and less tendency (for the structure of the embryo is mostly unimportant) to modify the young: and hence we might expect to find at this period similarities preserved between different groups of species which had been obscured and quite lost in the full-grown animals. I conceive on the view of separate creations it would be impossible to offer any explanation of the affinities of organic beings thus being plainest and of the greatest importance at that period of life when their structure is not adapted to the final part they have to play in the economy of nature. Order in time in which the great classes have first appeared. It follows strictly from the above reasoning only that the embryos of (for instance) existing vertebrata resemble more closely the embryo of the parent-stock of this great class than do full-grown existing vertebrata resemble their full-grown parent1 Origin, Ed. i. p. 441, vi. p. 607. 2 Origin, Ed. i. p. 449, vi. p. 617. stock. But it may be argued with much probability that in the earliest and simplest condition of things the parent and embryo must have resembled each other, and that the passage of any animal through embryonic states in its growth is entirely due to subsequent variations affecting only the more mature periods of life. If so, the embryos of the existing vertebrata will shadow forth the full-grown structure of some of those forms of this great class which existed at the earlier periods of the earth's history1: and accordingly, animals with a fish-like structure ought to have preceded birds and mammals; and of fish, that higher organized division with the vertebræ extending into one division of the tail ought to have preceded the equal-tailed, because the embryos of the latter have an unequal tail; and of Crustacea, entomostraca ought to have preceded the ordinary crabs and barnacles-polypes ought to have preceded jelly-fish, and infusorial animalcules to have existed before both. This order of precedence in time in some of these cases is believed to hold good; but I think our evidence is so exceedingly incomplete regarding the number and kinds of organisms which have existed during all, especially the earlier, periods of the earth's history, that I should put no stress on this accordance, even if it held truer than it probably does in our present state of knowledge. 1 Origin, Ed. i. p. 449, vi. p. 618. CHAPTER IX ABORTIVE OR RUDIMENTARY ORGANS The abortive organs of naturalists. PARTS of structure are said to be "abortive," or when in a still lower state of development "rudimentary1," when the same reasoning power, which convinces us that in some cases similar parts are beautifully adapted to certain ends, declares that in others they are absolutely useless. Thus the rhinoceros, the whale2, etc., have, when young, small but properly formed teeth, which never protrude from the jaws; certain bones, and even the entire extremities are represented by mere little cylinders or points of bone, often soldered to other bones: many beetles have exceedingly minute but regularly formed wings lying under their wing-cases, which latter are united never to be opened: many plants have, instead of stamens, mere filaments or little knobs; petals are reduced to scales, and whole flowers to buds, which (as in the feather hyacinth) never expand. Similar instances are almost innumerable, and are justly considered wonderful: probably not one organic being exists in which some part does not bear the stamp of inutility; for what can be clearer, as far as our reasoning powers 1 In the Origin, Ed. i. p. 450, vi. p. 619, the author does not lay stress on any distinction in meaning between the terms abortive and rudimentary organs. 2 Origin, Ed. i. p. 450, vi. p. 619. 3 Ibid. This argument occurs in Origin, Ed. i. p. 451, vi. p. 619. can reach, than that teeth are for eating, extremities for locomotion, wings for flight, stamens and the entire flower for reproduction; yet for these clear ends the parts in question are manifestly unfit. Abortive organs are often said to be mere representatives (a metaphorical expression) of similar parts in other organic beings; but in some cases they are more than representatives, for they seem to be the actual organ not fully grown or developed; thus the existence of mammæ in the male vertebrata is one of the oftenest adduced cases of abortion; but we know that these organs in man (and in the bull) have performed their proper function and secreted milk: the cow has normally four mammæ and two abortive ones, but these latter in some instances are largely developed and even (??) give milk1. Again in flowers, the representatives of stamens and pistils can be traced to be really these parts not developed; Kölreuter has shown by crossing a diæcious plant (a Cucubalus) having a rudimentary pistil2 with another species having this organ perfect, that in the hybrid offspring the rudimentary part is more developed, though still remaining abortive; now this shows how intimately related in nature the mere rudiment and the fully developed pistil must be. Abortive organs, which must be considered as useless as far as their ordinary and normal purpose is concerned, are sometimes adapted to other ends3: thus the marsupial bones, which properly serve to support the young in the mother's pouch, are present in the male and serve as the fulcrum for muscles connected only with male functions: in the 1 Origin, Ed. i. p. 451, vi. p. 619, on male mammæ. In the Origin he speaks certainly of the abortive mammæ of the cow giving milk,—a point which is here queried. 2 Origin, Ed. i. p. 451, vi. p. 620. The care of rudimentary organs adapted to new purposes is discussed in the Origin, Ed. i. p. 451, vi. p. 620. |