Origin of Species Variorum

Comparison with 1869

generated. It has, also, recently been ascertained that the cuckoo occasionally lays her eggs on the bare ground, sits on them and feeds her young; this rare and strange event evidently is a case of reversion to the long-lost aboriginal instinct of nidification. ↑

It has been objected by some authors that I have not noticed other related instincts and points of structure in the cuckoo, which are falsely spoken of as necessarily co-ordinated. But in all cases, speculation on any instinct or character known in only a single species, is useless, for we have no facts to guide us. Until quite recently the instincts of the European and of the non-parasitic American cuckoo alone were known; now, owing to Mr. E. Ramsay's observations, we know something about three Australian species, which lay their eggs in other birds' nests. The chief points referred to are three: firstly, that the cuckoo, with rare exceptions, lays only one egg in a nest, so that the large and voracious young cuckoo receives ample food. Secondly, that the egg is so remarkably small, that it does not exceed in size that of the skylark,—a bird not more than one-fourth of the size of the cuckoo; that this is a real case of adaptation we may infer from the fact of the non-parasitic American cuckoo laying eggs of full size proportionally with her body. Thirdly and lastly, that the young cuckoo, soon after birth, has the instinct, the strength, and a properly shaped back for ejecting its foster-brothers, which then perish from cold and hunger. This, it has been boldly maintained, is beneficently designed, in order that the young cuckoo may get sufficient food, and that its foster-brothers may perish, before, as it is supposed, they have acquired much feeling!

Turning now to the Australian species; though these birds generally lay only one egg in a nest, it is not rare to find two and even three eggs of the same species of cuckoo in the same nest. In the Bronze cuckoo the eggs vary greatly in size, from eight to ten lines in length. Now if it had been of any advantage to this species to have laid eggs even smaller than those now laid by her, so as to have deceived certain foster-parents, or, as is more probable, to have been hatched within some shorter period (for it is asserted that there is a relation between the size of eggs and the period of incubation), then there is no difficulty in believing that a race or species might have been formed which would have laid smaller and smaller eggs; for these would have been more safely hatched and reared. Mr. Ramsay remarks that two of the Australian cuckoos, when they lay their eggs in an open or not domed nest, manifest a decided preference for nests containing eggs similar to their own. The European species certainly manifests some tendency towards a similar instinct, but not rarely departs from it, as is shown by her laying her dull and pale-coloured eggs in the nest of the Hedge-warbler with its bright greenish-blue eggs: had she invariably displayed the above instinct, it would assuredly have been added to those which it is assumed must all have been acquired together. The eggs of the Australian Bronze cuckoo vary, according to Mr. Ramsay, in an extraordinary manner in colour; so that in this respect, as well as in size, natural selection assuredly might have secured and fixed any advantageous variation. ↑

With respect to the last point insisted on—namely, of the young European cuckoo ejecting its fosterbrothers—it must first be remarked that Mr. Gould, who has paid particular attention to this subject, is convinced that the belief is an error; he asserts that the young foster-birds are generally ejected during the first three days, when the young cuckoo is quite power less; he maintains that the young cuckoo exerts, by its hunger-cries, or by some other means, such a fascination over its foster-parents, that it alone receives food, so that the others are starved to death, and are then thrown out, like the egg-shells or the excrement, by the old birds. He admits, however, that the young cuckoo when grown older and stronger may have the power, and perhaps the instinct, of ejecting its foster-brothers, if they happen to escape starvation during the first few days after birth. Mr. Ramsay has arrived at a similar conclusion with respect to the Australian species, which he especially observed: he states that the young cuckoo is at first a little helpless fat creature, but, "as it grows rapidly, it soon fills up the greater part of the nest, and its unfortunate companions, either smothered by its weight, or starved to death through its greediness, are thrown out by their parents." Now, if it had been of great importance to the young cuckoo to have received as much food as possible during the first few days after birth, I can see no especial difficulty, if it possessed sufficient strength, in its gradually acquiring, during successive generations, the habit (first, perhaps, through mere unintentional restlessness) and the structure best fitted for ejecting its foster-brothers; for those young cuckoos which had such habits and structure would have been the best fed and most securely reared. ↑ I can see no more difficulty in this, than in young birds acquiring the instinct and the temporary hard tips to their beaks for breaking through their own shells;—or than in the young snake having in its upper jaw, as Owen has remarked, a transitory sharp tooth for cutting through the tough egg-shell. For if each part is liable to variation at any age, and the variations tend to be inherited at a corresponding age,—propositions which cannot, as we shall hereafter see, rightfully be disputed,—then the instincts and structure of the young can be slowly modified as well as those of the adult, and both cases must stand or fall together with the whole theory of natural selection.

The occasional habit of birds laying their eggs in other birds nests, either of the same or of distinct species, is not very uncommon with the Gallinaceæ; and this perhaps explains the origin of a singular instinct in the nearest allied group, that of ostriches. For several hen ostriches unite and lay first a few eggs in one nest and then in another; and these are hatched by the males. This instinct may probably be accounted for by the fact of the hens laying a large number of eggs, but, as in the case of the cuckoo, at intervals of two or three days. The instinct, however, of the American ostrich has not as yet been perfected; for a surprising number of eggs lie strewed over the plains, so that in one day's hunting I picked up no less than twenty lost and wasted eggs.

Many bees are parasitic, and regularly lay their eggs in the nests of .. other kinds of bees. This case is more remarkable than that of the cuckoo; for these bees have not only had their instincts but their structure modified in accordance with their parasitic habits; for they do not possess the pollen-collecting apparatus which would have been indispensable if they had .. stored food for their own young. Some species, likewise, of Sphegidæ (wasp-like insects) are parasitic in the same manner on other species; and M. Fabre has lately shown good reason for believing that, although the Tachytes nigra generally makes its own burrow and stores it with paralysed prey for its own larvæ, ... yet that, when this insect finds a burrow already made and stored by another sphex, it takes advantage of the prize, and becomes for the occasion parasitic. In this case, as with the supposed case of the cuckoo, I can

generated. I may add that, according to Dr. Gray and to some other observers, the European cuckoo has not utterly lost all maternal love and care for her own offspring. ↑ ↑

The occasional habit of birds laying their eggs in other birds nests, either of the same or of a distinct species, is not very uncommon with the Gallinaceæ and this perhaps explains the origin of a singular instinct in the allied group of ostriches. For several hen ostriches, at least in the case of the American species, unite and lay first a few eggs in one nest and then in another; and these are hatched by the males. This instinct may probably be accounted for by the fact of the hens laying a large number of eggs; but, as in the case of the cuckoo, at intervals of two or three days. This instinct, however, of the American ostrich has not as yet been perfected; for a surprising number of eggs lie strewed over the plains, so that in one days hunting I picked up no less than twenty lost and wasted eggs.

Many bees are parasitic, and always lay their eggs in the nests of bees of other kinds. This case is more remarkable than that of the cuckoo; for these bees have not only their instincts but their structure modified in accordance with their parasitic habits; for they do not possess the pollen-collecting apparatus which would be necessary if they had to store food for their own young. Some species, likewise, of Sphegidæ (wasp-like insects) are parasitic on other species; and M. Fabre has lately shown good reason for believing that although the Tachytes nigra generally makes its own burrow and stores it with paralysed prey for its own larvæ to feed on, yet that when this insect finds a burrow already made and stored by another sphex, it takes advantage of the prize, and becomes for the occasion parasitic. In this case, as with the supposed case of the cuckoo, I can