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condition, a heap of similar cells, is found in Pandorina, a colony of similar flagellate cells, all alike and living together in a common capsule. The third condition is found in Salinella, one of the most recent discoveries, and one of the most remark

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Amphioxus, showing early stages in development.

I. to IV.-Segmentation of the egg.
V.-Tubular stage, with an opening at each end.
VI.-Blastula stage, consisting of a hollow ball of cells.
The adult Amphioxus is shown at the top of the figure.

VI

able animals known, which is found in water containing 2 per cent. of salt. This organism consists of a tube, open at both ends, the wall of which is formed of a single layer of cells. The fourth condition, that of a hollow ball or blastula, is represented by Volvox, the well-known fresh-water

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A

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organism, which consists of a hollow ball formed by a single layer of flagellate cells like monads. (Fig. 21.)

It must be noted that these examples do not form

FIG. 21.

Adult organisms resembling early embryonic stages in higher animals.
A, Monad; B, Pandorina; C, Salinella; D, Volvox.

a continuous series in the sense of being derived
from one another. They are mentioned merely
with the object of showing that the very earliest
stages of development, like the later ones, may be
recapitulatory; and that these early larval stages

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represent possible forms of adult beings (whether animals or plants it is impossible to say) capable of independent existence; and they illustrate very forcibly the interest which attaches to embryology in the light of the Recapitulation theory, coupled with the theory of Evolution with modification.

perate and cold countries, yet the majority of tropical birds are dull-coloured, and in some groups the most brightly-coloured members are not tropical: for instance, the Arctic ducks and divers are more handsome than the tropical ones. The hummingbirds found in the Andes form another instance, for here they are confined to lofty mountains, sometimes to a particular mountain, "just beneath the line of perpetual snow, at an elevation of some 16,000 feet, dwelling in a world of almost constant hail, sleet, and rain." Again, most tropical and brightly-coloured birds are denizens of the forests, and shaded from the direct action of the sun; they abound also near the equator, where cloudy skies are very prevalent. In the case of flowers, Wallace remarks that "in proportion to the whole number of species of plants, those having gaily-coloured flowers are actually more abundant in the temperate zones than between the tropics."

NON-SIGNIFICANT COLOURS.

Many colours are the incidental results of chemical and physical structure, for the same reason that sulphate of copper is blue. The red colour of blood, the white colour of fat, the silvery colour of the bladder of many fish, the pigmented condition of the frog's peritoneum, and the green tint of the bones of many fish, are examples. The brilliant and varied colours of deep-sea animals are probably devoid of any signification, and the green colour of grass and the blue colour of the sky, for all we know, are non-significant.

The red colour of Tubifex, for example, is associated with the physiological activity of hæmoglobin ; the red colour here is probably disadvantageous as such, but is counterbalanced by the physiological utility of the pigment for respiratory purposes. A similar explanation holds with chlorophyll, the green colouring matter of plants.

It is important to note that red is only red in the presence of light, and that a red animal if put in a dark place ceases to be red; or if put in a green light, which it is incapable of reflecting. Non-significant colours "form the material out of which natural or sexual selection can form significant colours," and "all animal colours must have been originally non-significant."

THE DIRECT ACTION OF ENVIRONMENT.

Distinct colour varieties occur locally among the Lepidoptera, and a great prevalence of green is shown by the fauna of Ceylon, not only by terrestrial forms, but by echinoderms, corals, and other animals. That differences in food have an effect on colour has been shown by feeding the larvæ of various kinds of Lepidoptera on different plants: the larvæ of the eyedhawk moth, the brimstone moth, and the peppered moth show changes of this kind. This effect is not due to the colour showing through the skin, but must be effected through the nervous system, the particular pigment being actually built up by the caterpillar.

Pupæ assume in many cases the colour of the

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