It is clear from the last few chapters that the theory of mimicry in butterflies with its interpretation of the building up of these likenesses by means of natural selection in the form of predaceous birds and other foes is open to destructive criticism from several points of view. The evidence from mimicry rings makes it almost certain that in some cases the resemblance must be founded on an initial variation of such magnitude that the mimic could straightway be confused with the model. Till the mimic can be mistaken for the model natural selection plays no part. The evidence from breeding suggests strongly that in certain cases (e.g. Papilio polytes) the likeness arose in the form in which we know it to-day. In such cases there is no reason for supposing that natural selection has had anything to do with the formation of the finished mimic. Considerations of this nature may be said to have destroyed the view, current until quite recently, that in the formation of a mimetic resemblance the exclusive agent was natural selection. During the past few years it has come to be admitted by the staunchest upholders of the theory of mimicry that natural
selection would not come into play until the would-be mimic was sufficiently like the model to be confused with it under natural conditions[[80]]. The part now often attributed to natural selection is to put a polish on the resemblance and to keep it up to the mark by weeding out those which do not reach the required standard. It is supposed that if natural selection ceases to operate the mimetic resemblance is gradually lost owing to the appearance of variations which are no longer weeded out. An interesting case has recently been brought forward by Carpenter[[81]] and explained on these lines: The Nymphaline Pseudacraea eurytus is a polymorphic species found in Central Africa. In Uganda it occurs in several distinct forms which were originally supposed to be distinct species. Three of these forms bear a marked resemblance to three species of the Acraeine genus Planema.
| Mimic | Model | |
| Pseudacraea eurytus | Planema | |
| Form | hobleyi[[82]] ([Pl. VII], figs. 6, 7) | macarista ([Pl. VII], fig. 2) |
| terra ([Pl. VII], fig. 8) | tellus ([Pl. VII], fig. 3) | |
| obscura | paragea ([Pl. VII], fig. 4) | |
These different species occur round Victoria Nyanza and also on some of the islands in the lake. Some
interesting points are brought out by a comparison of the occurrence and variation of the species on the mainland with what is found on Bugalla Island in the Sesse Archipelago. On the mainland the Pseudacraeas are abundant but the Planemas even more so, outnumbering the former by about 5:2[[83]]. Moreover, it is rare to find individuals more or less intermediate between the three forms, though they are known to occur. On Bugalla Island, however, a different state of things is found. The Pseudacraeas are very abundant, whereas the Planemas, owing doubtless to the scarcity of their food plant, are relatively rare, and are very greatly outnumbered by the Pseudacraeas. At the same time the proportion of transitional forms among the Pseudacraeas is definitely higher than on the mainland. These facts are interpreted by Carpenter as follows:—
On the mainland where the models are abundant there is a vigorous action on the part of natural selection. The mimetic forms have a strong advantage and the non-mimetics have been gradually weeded out. But on the island, where the Pseudacraeas outnumber the models, the advantage obtained through mimicry is not so great. The so-called transitional forms are little, if at all, worse off than those closely resembling the scarce models, and consequently have as good a chance of surviving as any of the typical mimetic forms. On
the mainland, however, the enemies of Pseudacraea are well acquainted with the Planema models which are here common, and discriminate against individuals which are not close mimics of the Planemas. The result is that on the mainland transitional forms are scarcer than on the island. Natural selection maintains a high standard for the mimetic likeness on the mainland owing to the abundance of the model; but when the model is scarce the likeness ceases to be kept up to the mark strictly, and tends to become lost owing to the appearance of fresh variations which are no longer weeded out.
Here it should be stated that the various Pseudacraeas form a population in which the different forms mate freely with one another. In the few breeding experiments that Dr Carpenter was able to make he found that obscura could produce terra, and that tirikensis was able to give obscura, the male in each case being, of course, unknown. Far too little work has as yet been done on the genetics of these various forms, and it would be rash to make assumptions as to the nature of the intermediates until the method of experimental breeding has been more extensively employed in analysing their constitution. Possibly it is not without significance that the abundance or scarcity of the obscura form runs parallel with the abundance or scarcity of the intermediates. It suggests that the intermediates are heterozygous in some factor for which the typical obscura is homozygous, and the fact that the intermediates are more numerous than
obscura is what is to be looked for in a population mating at random. This case of the polymorphic Pseudacraea eurytus is one of the greatest interest, but it would be hazardous to draw any far-reaching deductions from such facts as are known at present. When the genetics of the various typical forms and of the intermediates has been worked out it will be disappointing if it does not throw clear and important light on these problems of mimetic resemblance.
As the result of modern experimental breeding work it is recognised that an intermediate form between two definite varieties may be so because it is heterozygous for a factor for which one variety is homozygous and which is lacking in the other—because it has received from only one parent what the two typical varieties receive from both parents or from neither. Its germ cells, however, are such as are produced by the two typical forms, and the intermediate cannot be regarded as a stage in the evolution of one variety from the other. In these cases of mimicry the existence of intermediate forms does not entail the deduction that they have played a part in the evolution of one pattern from another under the influence of a given model. It is quite possible that the new mimetic pattern appeared suddenly as a sport and that the intermediates arose when the new form bred with that which was already in existence. But before we are acquainted with the genetic relationships between the various forms, both types and intermediates, speculation as to their origin must remain comparatively worthless.