The science of life : fully illustrated in tone and line and including many diagrams

BOOK 3

be explained unless we suppose that dardanus is descended from ‘an ordinary Swallowtail with tails in both sexes.

Sometimes one only of a pair of organs becomes vestigial. Female birds have only one ovary and oviduct—the left—doubtless to provide against the accidents that might occur if two large and brittle eggs were to knock about simultaneously in their insides ; but the right ovary and oviduct are always present as miniature and useless vestiges. The reptiles, from which birds undoubtedly have evolved, possess a pair of functional ovaries and oviducts: so that the presence of the vestigial right set of organs is perfectly intelligible to the Evolutionist.

Vestiges may, of course, also occur in plants. The well-known Butcher’s Broom (Ruscus) gives us an example. In this plant, what appear to be the leaves are really flattened-out stems, as is shown by the fact that on them are born the flowers and by other anatomical details. This curious arrangement is an adaptation to a dry soil ; the leaf-like stems are flattened vertically, instead of horizontally like ordinary leaves, and are accordingly not so much heated as leaves would be, and so lose less water from their pores. However, leaves are not absent in the Butcher’s Broom ; they are still to be found, but only in the form of vestiges, mere scale-like organs below the leaf-like stems and the flower-stalks. These contain very little chlorophyll, and in any case soon wither and fall off, so that they are quite useless for the leaf’s prime function of foodmanufacture. Very similar vestiges of leaves, though often still more reduced in size, are to be found in many of the cactuses and prickly pears, which, too, have taken on the function of food-manufacture by their thickened stems, and for the same reason of economizing water.

In flowers, too, vestigial organs may be found. The flower of the common figwort, Scrophularia, has changed from its original five-rayed symmetry, so common among flowers, to a bilateral arrangement. And, of its five stamens, four are grouped in two pairs, and the odd fifth never develops any pollen; it is purely vestigial, and quite useless. In the related Gratiola, only two of the stamens produce pollen, one has been entirely lost, and two have been reduced to vestiges.

These flowers have lost their original, perfect, five-rayed symmetry ; and in the process some of the stamens have become useless. But instead of being discarded they

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sometimes linger on as vestiges to help the botanist unravel the plant’s past evolution.

And so we might proceed. There are the sightless vestiges of eyes in many cave-fishes and cave-shrimps; the feeble vestiges of wings in kiwi and dodo, and in many flightless insects ; the poor useless limbs of Proteus, and other newt-like creatures, which have preferred swimming to crawling, and of some lizards which have taken to a burrowing life; and there are a host of examples which man carries about with him in his own person. But we are reserving for a later section that museum of evolutionary

biology, the human body.

$73 The Evidence of the Embryo

Vestigial organs, actually so diminutive, swell in their theoretical aspect to mountainous proportions, forming impossible barriers to the attacks of the Anti-Evolutionist. But obstacles almost or quite as formidable await him in the facts of embryology.

About a hundred years ago, von Baer, the great embryologist, omitted to label some specimens of embryos which he had put away in spirit. When he came to examine them later he found but we will quote his own words :—‘ I am quite unable to say to what class they belong. They may be lizards, or small birds, or very young mammals, so complete is the similarity in the mode of formation . . . ofall these animals.” Thinking over this he came to formulate a general law—that animals resembled each other more and more the farther back we pursued them in development. This law in general holds good, and this resemblance of embryos or larve is a very striking fact, very difficult to explain save on evolutionary lines. A child of two can tell a pig from a man, a hen from a monkey, an elephant from a snake. But these animals are only easy to tell apart in the later stages of their development. When they were early embryos, they were all so alike that not merely the ayerage man but the average biologist would not be able to distinguish them, and even a specialist in embryology might be pardoned a mistake (Fig. 136).

But this is by no means all. “The embryos of different animals, in addition to being more like each other as development is traced backwards, show also a widening contrast with their parents and their adult destiny. They become unlike their adult selves, but at the same time and in the same respects their construction comes to resemble