Animal Intelligence / The International Scientific Series, Vol. XLIV.

by George John Romanes

Having said this much, and believing as I do the Doctor to be partly justified in the carefully worded enunciation of what he calls a 'law of nature,' I must now declare that it is only 'approximately,' and by no means universally true that the cuckoo's egg is coloured like those of the victims of her imposition, &c.

Still, when so great an authority as Professor Newton expresses himself satisfied that there is a marked tendency to such imitation, which in some cases leads to extraordinary variations in the colouring of the cuckoo's egg, the alleged fact becomes one which demands notice. The question, of course, immediately arises, How is it conceivable that the fact, if it is a fact, can be explained? We cannot imagine the cuckoo to be able consciously to colour her egg during its formation in order to imitate the eggs among which she is about to lay it; nor can we suppose that having laid an egg and observed its colouring, she then carries it to the nest of the bird whose eggs it most resembles. Professor Newton suggests another theory, which he seems to think sufficient, but which I confess seems to me little more satisfactory than the impossible theories just stated. He says:—

Only one explanation of the process can, to my mind, be offered. Every person who has studied the habits of birds with sufficient attention will be conversant with the tendency which certain of those habits have to become hereditary. It is, I am sure, no violent hypothesis to suppose that there is a very reasonable probability of each cuckoo most commonly placing her eggs in the nest of the same bird, and of this habit being transmitted to her posterity.

Now it will be seen that it requires but only an application to this case of the principle of 'natural selection,' or 'survival of the fittest,' to show that if my argument be sound, nothing can be more likely than that, in the course of time, that principle should operate so as to produce the facts asserted, the eggs which best imitated those of particular foster-parents having the best chance of duping the latter, and so of being hatched out.

Now, granting to this hypothesis the assumption that individual cuckoos have special predilections as to the species in whose nests they are to lay their eggs, and that some of these species require to be deceived by imitative colouring of the egg to prevent their tilting it out, there is still an enormous difficulty to be met. Supposing that one cuckoo out of a hundred happens to lay eggs sufficiently like those of the North African magpies (a species alluded to by Professor Newton) to deceive the latter into supposing the egg to be one of their own. This I cannot think is too small a proportion to assume, seeing that, ex hypothesi, the resemblance must be tolerably close, and that the egg of the magpie does not resemble the great majority of eggs of the cuckoo. Now, in order to sustain the theory, we must suppose that the particular cuckoo which happens to have the peculiarity of laying eggs so closely resembling those of the magpie, must also happen to have the peculiarity of desiring to lay its eggs in the nest of a magpie. The conjunction of these two peculiarities would, I should think, at a moderate estimate reduce the chances of an approximately coloured egg being laid in the appropriate nest to at least one thousand to one. But supposing the happy accident to have taken place, we have next to suppose that the peculiarity of laying these exceptionably coloured eggs is not only constant for the same individual cuckoo, but is inherited by innumerable generations of her progeny; and, what is much more difficult to grant, that the fancy for laying eggs in the nest of a magpie is similarly inherited. I think, therefore, notwithstanding Professor Newton's strong opinion upon the subject, that the ingenious hypothesis must be dismissed as too seriously encumbered by the difficulties which I have mentioned. We may with philosophical safety invoke the influence of natural selection to explain all cases of protective colouring when the modus operandi need only be supposed simple and direct; but in a case such as this the number and complexity of the conditions that would require to meet in order to give natural selection the possibility of entrance, seem to me much too considerable to admit of our entertaining the possibility of its action—at all events in the way that Professor Newton suggests. Therefore, if the facts are facts, I cannot see how they are to be explained.

Cuckoos are not the only birds which manifest the parasitic habit of laying their eggs in other birds' nests.

Some species of Melothrus, a widely distinct genus of American birds, allied to our starlings, have parasitic habits like those of the cuckoo; and the species present an interesting gradation in the perfection of their instincts. The sexes of Melothrus cadius are stated by an excellent observer, Mr. Hudson, sometimes to live promiscuously together in flocks and sometimes to pair. They either build a nest of their own, or seize on one belonging to some other bird, occasionally throwing out the nestlings of the stranger. They either lay their eggs in the nest thus appropriated, or oddly enough build one for themselves on the top of it. They usually sit on their own eggs and rear their own young; but Mr. Hudson says it is probable that they are occasionally parasitic, for he has seen the young of this species feeding old birds of a distinct kind and clamouring to be fed by them. The parasitic habits of another species of Melothrus, the M. Canariensis, are much more highly developed than those of the last, but are still far from perfect. This bird, as far as it is known, invariably lays its eggs in the nests of strangers, but it is remarkable that several together sometimes commence to build an irregular untidy nest of their own, placed in singularly ill-adapted situations, as on the leaves of a large thistle. They must, however, as far as Mr. Hudson has ascertained, complete a nest for themselves. They often lay so many eggs, from fifteen to twenty, in the same foster-nest, that few or none can possibly be hatched. They have, moreover, the extraordinary habit of pecking holes in the eggs, whether of their own species or of their foster-parents, which they find in the appropriated nests. They drop also many eggs on the bare ground, which are thus wasted. A third species, the M. precius of North America, has acquired instincts as perfect as those of the cuckoo, for it never lays more than an egg in a foster-nest, so that the young bird is securely reared. Mr. Hudson is a strong disbeliever in evolution, but he appears to have been so much struck by the imperfect instincts of the Melothrus Canariensis that he quotes my words, and asks, 'Must we consider these habits not as especially endowed or created instincts, but as small consequences of one general law, namely transition?'[177]

Such are all the facts and considerations which I have to present with reference to the curious instinct in question. It will be seen that—with one doubtful or not sufficiently investigated exception, viz., that of cuckoos adapting the colour of their eggs to that of the eggs of the foster-parents—there is nothing connected with these instincts that presents any difficulty to the theory of evolution. We may, perhaps, at first sight wonder why some counteracting instinct should not have been developed by the same agency in the birds which are liable to be thus duped; but here we must remember that the deposition of a parasitic egg is, comparatively speaking, an exceedingly rare event, and therefore not one that is likely to lead to the development of a special instinct to meet it.

General Intelligence.

Under this heading I shall here, as in the case of this heading elsewhere, string together all the instances which I have met with, and which I deem trustworthy, of the display of unusually high intelligence in the class, family, order, or species of animals under consideration—the object of this heading in all cases being that of supplying, by the facts mentioned beneath it, a general idea of the upper limit of intelligence which is distinctive of each group of animals.