are bright-red in colour. Their presence is possibly in relation to the fact that some species are active swimmers.

Before speaking of the camera eyes of Vertebrates, it may be well to mention certain simpler visual structures which are found in some of the most primitive members of that group. In the tadpole larva of a Sea-Squirt there is a simple cup-like directioneye formed by thickening of the wall of the brain, and projecting into that organ (see fig. 1049, p. 38). Since the larva is transparent light-rays are able to reach it. The adult condition results from a remarkable series of modifications (see vol. iii, p. 421), which include simplification of the nervous system with loss of the brain-eye and brainotocyst. The only compensation for this loss of vision consists in the appearance of a circlet of pigmented eye-spots round the openings by which currents of seawater enter and leave the body.

lp.

m.

ph.

n.

2.

.py.

Fig. 1060.-Diagrammatic Cross Section through the Head of a Tadpole, to Illustrate the Development of the Eyes, enlarged. f., Fore-brain; r. and r., retina and its external pigment-layer; lp., lens-pit; ., lens; a., an artery; m.,

mouth cavity; n., a nerve; pl., pharynx;

py., pituitary body.

The visual organs of the transparent Lancelet (Amphioxus) are of even simpler kind. The so-called "eye" is merely a deeply- pigmented spot in the extreme front end of the nerve-tube, and there is in addition a series of similar but smaller spots in the floor of the nervetube behind the head-region.

The facts just mentioned prepare us for the statement that the ordinary camera eyes of Fishes and still higher Vertebrates are partly derived from the brain, and in this they differ from the camera eyes of Invertebrates, which are of epidermic nature. Two stages in the development of the Vertebrate eye are represented in fig. 1060. From either side of the fore-brain of the embryo an optic vesicle grows out towards the ectoderm, in which a corresponding pit makes its appearance. The end of the vesicle becomes as it were pushed in to form a double-walled optic cup, of which the inner and thicker layer is destined to produce the greater part of the retina, or sensitive eye-screen, while the outermost pigmented layer of this is derived from the outer part of the cup. The external ectodermic pit closes, and is pinched off as a vesicle, which lies in the optic cup (see right-hand side of figure), and ultimately thickens into the lens. The stalk of the optic cup becomes the

optic nerve. Since the brain itself is of ectodermic origin (see p. 20), it is clear that the parts of the eye so far mentioned are all derived from ectoderm. The rest of the eyeball, including its two outer coats and refracting contents (see vol. i, p. 57), are formed from the middle embryonic layer (mesoderm). This curious kind of development clearly suggests that in the remote ancestors of Vertebrates the eyes were internal projections from the brain, and received their light through the transparent tissues external to them, as is still the case in the single eye of the tadpole of a SeaSquirt. The free ends of the visual cells (rods and cones) were directed towards the cavity of the brain. As in the course of evolution the brain

1.

0.n.

became more and more complex, an opaque skull was developed for its protection, and the brain-eyes, having their supply of light thus cut off, were obliged, so to speak, to grow outwards. Subsequently they were improved into camera eyes by the development of a lens. Further improvements consisted in the evolution of eye - muscles, eyelids, and complex focussing arrangements. The visual cells (rods and cones) of the Vertebrate eye present the remarkable peculiarity of pointing away from the light, one result of the manner in which the retina is developed. In Vertebrates, such as Fishes, which have to see under water, the lens of the eye is spheroidal, and one mark of the aquatic ancestry of the Amphibia is the possession of a lens of similar shape. But thoroughgoing land Vertebrates have lost this primitive character, for in them the lens is more or less flattened and biconvex, as an adaptation to seeing in air.

Fig. 1061. Section through the Pineal Eye of a Tuatara (Hatteria), enlarged. f., Fibrous covering; 7, lens; r., ., retina; b., blood-vessel; o.n., optic nerve.

An extremely interesting and remarkable arrangement is found in certain bony fishes known as Double-Eyes (Anableps), native to the coasts and estuaries of tropical America. The name has been given because either eye, as seen from the exterior, is marked off into upper and lower halves by a dark transverse band. Dissection shows that the upper half of the lens is biconvex, and the lower half spheroidal. And since these fishes habitually swim at the surface, with only the lower part of the eye immersed, we can

only conclude that this half can see clearly in water, while the upper half has been so modified that distinct vision in air has also become possible.

Some of the Reptiles possess a more or less degenerate third or pineal eye on the top of the head (fig. 1061). It is connected with the roof of the 'twixt-brain. There seems good reason to believe that the ancestral Vertebrates had at least one visual organ in this position, probably serving as a means of detecting enemies attacking from above, a contingency to which aquatic forms are peculiarly liable. We may perhaps compare it with the internal brain-eye of the Ascidian tadpole, which also is unpaired and dorsal.

ANIMAL INSTINCT AND INTELLIGENCE

CHAPTER LIX

GENERAL PRINCIPLES-INSTINCT AND INTELLIGENCE IN HIGHER INVERTEBRATES AND VERTEBRATES

GENERAL PRINCIPLES

Having briefly surveyed the salient facts regarding the Nervous System and Sense-Organs we naturally pass on to the consideration of those higher manifestations of life known as Instinct and Intelligence, which play a very important part in the adjustment of animals to their surroundings. To do anything like full justice to the subject at least half a volume would be required, and it is only possible here to attempt a brief summary of general principles, adding to this a few typical illustrations. Many other examples, however, will be found in other parts of this book. As regards the present section, the writer wishes to acknowledge his great indebtedness to the works of Principal Lloyd Morgan, i.e. Habit and Instinct, Animal Life and Intelligence, and Animal Behaviour, to which are referred those readers who wish further information on this branch of zoology.

Something has already been said about Reflex Actions (see p. 9), which are comparatively simple responses to external stimuli. In very lowly animals, such as Animalcules (Protozoa), these, together with equally simple spontaneous actions, are sufficient to meet all the contingencies of existence. So apparently purposeful, however, are many of these actions, that some observers are inclined to ascribe mental powers to such forms. Either to prove or to disprove such a view is impossible, for we have no direct knowledge of the mind of any animal save Man, and can only make more or less probable guesses about other forms. We may feel pretty sure, however, that the evolution of the nervous

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system through increasingly complex stages has been associated with a corresponding evolution of mind, and there is considerable justification for doubting whether animals devoid of a nervous system, or possessed of a very imperfect one, are endowed with more than a dim consciousness or awareness of existence, or are capable of manifesting either Instinct or Intelligence.

An animal which inherits the power of performing more or less complex actions helping to adjust it to its surroundings, independently of experience or instruction, is said to display Instinct, and such actions may be termed instinctive. They differ from Reflex Actions in being more elaborate, and many of them are partly or entirely spontaneous. But our knowledge is at present too incomplete to enable us to draw the line between actions which are of reflex character and those which are instinctive. It is only when dealing with the higher Invertebrates and the Vertebrates that we can use the latter term with any degree of certainty. The Birch-Weevil (see vol. iii, p. 394), for instance, certainly displays instinct when she constructs an elaborate leaf-funnel for the reception of her eggs. This very complicated piece of work is performed, so far as we know, with unerring certainty and without previous experience. Nor can the weevil have more than a hazy knowledge of the purpose of her work, which is probably done quite mechanically.

An animal is said to show Intelligence when it profits by experience, accommodating its actions to the exigencies of changed or changing surrounding. There is an inherited basis to such actions; it is the controlling power which makes them intelligent. The difference between Instinct and Intelligence is explained with admirable lucidity in the following passage from Lloyd Morgan (in Animal Behaviour):—“ Such an animal as a newly-hatched bird or an insect just set free from the chrysalis is a going concern, a living creature. It is the bearer of wonderfully complex automatic machinery, capable, under the initiating influence of stimuli, of performing instinctive acts. But if this were all, we should have no more than a cunningly-wrought and self-developing automatic machine. What the creature does instinctively at first it would do always, perhaps a little more smoothly as the organic mechanism settled down to its work-just as a steam-engine goes more smoothly when it has been running for a while; but otherwise the action would continue unchanged. Instinctive behaviour would