Hence if we can show, as we believe we can, that his arguments are not sound, we may take it that we have demonstrated that the theory in its present form is untenable. It is worthy of notice that Professor Poulton sets forth three other suggestions which have been proposed as substitutes for natural selection as an explanation of the phenomena of mimicry. The first is the theory of External Causes, namely, that the resemblance is due to some external cause, such as food or climate. The second is the theory of Internal Causes, which states that mimetic resemblance is due to internal developmental causes. The third is the suggestion that sexual selection has caused the origin of these resemblances. He then proceeds to demolish these to his own satisfaction, and adds triumphantly, “The conclusion appears inevitable that under no theory, except natural selection, do the various resemblances of animals to their organic and inorganic environments fall together into a natural arrangement and receive a common explanation” (p. 228). To reasoning of this description there is an obvious reply. Even if it be granted that the alternatives to the theory of natural selection as set forth by Professor Poulton are untenable, it does not follow that natural selection affords an adequate explanation. If A, B, C and D are charged with theft and the prosecutor proves that neither A nor B nor C committed the theft, this will not suffice to secure the conviction of D. It is quite possible that a fifth person, E, may be the culprit. Much of the popularity of the theory of natural selection is due to the fact that biologists have not yet been able to discover a substitute for it. It seems to us that the proper method of making progress in science is not to bolster up natural selection by ingenious speculations, but to look around for other hitherto undiscovered causes. [Illustration: KING-CROW OR DRONGO] [Illustration: DRONGO-CUCKOO] Objections to the Theory that the so-called Cases of Mimicry owe their Origin to Natural Selection It is obvious that for one creature to resemble another can be of little or no benefit to either until the resemblance is tolerably close. It is, therefore, insufficient to prove the utility of the perfected resemblance. We may readily grant this and yet maintain that the origin of the resemblance cannot be due to the action of natural selection. The Drongo-cuckoo (_Surniculus lugubris_) displays so great a likeness to the King Crow (_Dicrurus ater_) that it is frequently held up by Neo-Darwinians as an excellent example of mimicry among birds. But D. Dewar writes, on page 204 of _Birds of the Plains_: “I do not pretend to know the colour of the last common ancestor of all the cuckoos, but I do not believe that the colour was black. What then caused _Surniculus lugubris_ to become black and assume a king-crow-like tail?