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228

EVIDENCE THAT DAPHNIAS

placing the yellow in another trough, as before, for comparison. The preference for the yellow was as marked as ever. In the experiments with the red and yellow the numbers were respectively

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When, therefore, the red solution was sufficiently light, the Daphnias were indifferent to it. In the experiments with light blue the numbers were

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One other possible objection also suggested itself to I thought it might be said that the Daphnias went under the yellow and the green not on account of any preference for yellow or green light, but on account of the shelter afforded by the covering. To test this, I covered one half of a trough over with transparent glass, leaving the other uncovered; but after twenty observations I found the number of Daphnias in each half to be practically identical. The mere fact of the covering, therefore, made no difference. In this way I was able to test the preference of the Daphnias for various colours, and the result made it abundantly clear that Daphnias have the power of distinguishing between light of different wave-lengths, and that they prefer the light which we call yellow and green. Whether it actually appears to them as it does to us is, of course,

PERCEIVE DIFFERENCES OF COLOR.

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another and a more difficult question-one, moreover, not yet solved even for the higher animals. Nor would I necessarily claim for them any aesthetic sense of beauty; it must be remembered that they feed on minute algæ and other minute vegetables, the prevalent colors of which are yellow, yellowish green, and green. There is, therefore, nothing improbable, à priori, but rather the reverse, in their preference for these colors.

It will be observed that though in these vessels the Daphnias made their preference unmistakable, there were always a certain number in the least popular part. This is natural, because, as the position of the light half was reversed every observation, the Daphnias had to swim across the vessel, and some naturally did not find their way to the favourite part. Then, again, in any considerable numbers of Daphnias some are changing, or have recently changed, their skin, and are, therefore, more or less inactive. Moreover, in pure water the desire for food must often overpower any preference for one colour over another. To such causes as these we must, I think, attribute the presence of so many Daphnias in the first vessel at the opaque end, and in the second in the uncovered part.

Still, it was of course not impossible that the presence, for instance, of a certain number under the red and blue was due to a difference of taste; that, though the majority preferred yellow, there might be some preferring blue or red. To test this I tried the following experiment. I placed, as before, fifty Daphnias in three of the vessels, covering one half of one with the yellow, of a second with blue, and the third with red. I then from time to time, at intervals of not less than half an hour, removed these which were in the un

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EVIDENCE THAT DAPHNIAS

covered part and replaced them with an equal number of fresh ones. If, then, some Daphnias preferred red or blue, I ought thus to eliminate the others, and gradually to get together fifty agreeing in this taste. This, however, was not the case. In the first experiment, an hour after the Daphnias were placed in the vessels there were, out of 50, 41 under the yellow, 16 under the red, and 15 under the blue, the remaining 9, 34, and 35 respectively being in the uncovered portions. These, then, I removed and replaced by others. After doing this five times, and thus adding 80 in the yellow division, 187 in the red, and 209 in the blue, the numbers were 37 under the yellow, 15 under the red, and 6 under the blue.

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In the second experiment, the numbers after the first hour were 32 under the yellow, 10 under the red, and 11 under the blue. After five observations, during which 86 were added to the yellow division, 188 to the red, and 180 to the blue, the numbers were-under the yellow, 35; red, 11; blue, 15.

In the third experiment, the numbers after half an hour were 40 under the yellow, 14 under the red, and 8 under the blue. After five observations, during which 73 were added to the yellow, 186 to the red, and 206 to the blue, there were-under the yellow, 43; under the red, 15; and under the blue, 7.

In the fourth experiment, the numbers after half an hour were 38 under the yellow, 15 under the red, and 14 under the blue. After six observations, during which 89 were added to the yellow, 166 to the red, and 176 to the blue, the numbers were under the yellow, 30; under the red, 19; and under the blue, 10.

In the fifth experiment, the numbers after half an hour were 40 under the yellow, 14 under the red, and

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13 under the blue. After seven observations, during which 86 were added to the yellow, 263 to the red, and 272 to the blue, the numbers were-under the yellow, 38; under the red, 13; and under the blue, 15.

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I conclude, then, that the presence of some of the Daphnias in the red, blue, and violet is more or less due to the causes above indicated, and not to any individual preference for those colors.

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My experiments, I think, show that, while the Daphnias prefer light to darkness, there is a certain maximum of brilliancy beyond which the light becomes inconveniently bright to them, and that they can distinguish between light of different wave-lengths. I suppose it would be impossible to prove that they actually perceive colours; but to suggest that the rays of various wave-lengths produce on their eyes a different impression from that of color, is to propose an entirely novel hypothesis.

At any rate, I think I have shown that they do distinguish between rays of different wave-lengths, and prefer those which to our eyes appear green and yellow.

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CHAPTER XI,

ON RECOGNITION AMONG ANTS.

DURING the many years that I have had ants under observation, I have never on any occasion seen anything like a quarrel between any two ants belonging to the same community. This is certainly very much to their credit. The experience of Huber, Forel, McCook, and others who have watched ants, is, moreover, the same as mine. I have also shown* that they recognize one another even after a separation of a year and nine months.

On the other hand, every community of ants is hostile to every other. I am not now speaking of ants belonging to different kinds, but of ants belonging to the same species. Some species, indeed, are more intolerant of strangers than others; but, as regards most species of ants, it may be said that if an individual be taken from its own nest and introduced into another, even though belonging to the same species, it will be at once attacked and driven, or rather dragged, out.

These facts, then, show that the ants of a community all recognize one another. But when we consider the immense number of ants in a nest, amounting in some cases to over 500,000, this is indeed a wonderful fact.

See "Ants, Bees, and Wasps."

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