Page:Popular Science Monthly Volume 15.djvu/29

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19
CLEWS IN NATURAL HISTORY.

tion has been extreme, but there can be no doubt that the ancestors of the beetles with modified wings possessed fully developed appendages; otherwise we must regard the order of nature as being one long string of strange and incoherent paradoxes. Mr. Darwin has given us some instructive hints regarding the modification of beetles' wings and feet in his remarks on the effects of the use and disuse of parts in the animal economy. Kirby, the famous authority on entomology, long ago noted the fact that, in the males of many of the dung-beetles, the front feet were habitually broken off. Mr. Darwin confirms the observation of Kirby, and further says that in one species (Onites apelles) the feet "are so habitually lost that the insect has been described as not having them." In the sacred beetle (Ateuchus) of the Egyptians, the tarsi are not developed at all. Mr. Darwin remarks that necessarily we can not, as yet, lay over-much stress upon the transmission of accidental mutilations from parent to progeny, although, indeed, there is nothing improbable in the supposition; and, moreover, Brown-Séquard noted that, in the young of Guinea-pigs which had been operated upon, the mutilations were reproduced. Epilepsy, artificially produced in these latter animals, is inherited by their progeny. "Hence," says Darwin, "it will perhaps be safest to look at the entire absence of the anterior tarsi (or feet) in Ateuchus, and their rudimentary condition in some other genera, not as cases of inherited mutilations, but as due to the effects of long-continued disuse; for, as many dung-feeding beetles are generally found with their tarsi lost, this must happen in early life; therefore the tarsi can not be of much importance, or be much used by these insects."

The beetles of Madeira present us with a remarkable state of matters, which very typically illustrates how rudimentary wings may have been produced in insects. Two hundred beetles, out of over five hundred species known to inhabit Madeira, are "so far deficient in wings that they can not fly." Of twenty-nine genera confined to the island, twenty-three genera include species wholly unable to wing their way through the air. Now, beetles are frequently observed to perish when blown out to sea; and the beetles of Madeira lie concealed until the storm ceases. The proportion of wingless beetles is said by Mr. Wollaston to be "larger in the exposed Desertas than in Madeira itself"; while most notable is the fact that several extensive groups of beetles which are numerous elsewhere, which fly well, and which "absolutely require the use of their wings," are almost entirely absent from Madeira. How may the absence of wings in the Madeiran beetles be accounted for? Let Mr. Darwin reply: "Several considerations make me believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection, combined probably with disuse. For during many successive generations each individual beetle which flew least, either from its wings having been ever so little less perfectly developed, or from indolent habit, will have had the best