The Making of Species

by Douglas Dewar; Frank Finn

“If,” writes Mr Tylor, “we take highly decorated species—that is, animals marked by alternate dark or light bands or spots, such as the zebra, some deer, or the carnivora, we find, first, that the region of the spinal column is marked by a dark stripe; secondly, that the regions of the appendages, or limbs, are differently marked; thirdly, that the flanks are striped or spotted, along or between the regions of the lines of the ribs; fourthly, that the shoulder and hip regions are marked by curved lines; fifthly, that the pattern changes, and the direction of the lines, or spots, at the head, neck, and every joint of the limbs; and, lastly, that the tips of the ears, nose, tail, and feet, and the eyes are emphasised in colour.”

More recently Mr J. Lewis Bonhote has devoted much attention to this important subject. The results of his researches are summarised on page 185 of vol. xxix. of the Proceedings of the Linnæan Society, and on page 258 of the Proceedings of the Fourth International Ornithological Congress, 1905. Mr Bonhote states that the presence or absence of colour tends almost invariably to make its appearance, first of all, on certain definite tracts, common to mammals and birds alike, which he calls pœcilomeres.

Pœcilomeres

“Pœcilomeres,” he writes, “are situated on the following parts, viz., chin, malar stripe, maxillary stripe, a spot above and slightly in front of the eye, a spot below or slightly behind the eye, the ear, crown of the head, occiput, fore-end of sternum, vent, rump, thighs, wrist, shoulders (above and below).

“Now, there is hardly any species of bird on which one or more of these pœcilomeres is not ‘picked out’ (to use a painter’s expression) in some colour different from that of the surrounding parts, and, in fact, most of the so-called recognition or protective markings will be found on these patches.

“On the other hand, among many species the differentiation of colour on the pœcilomeres is not so conspicuous as to attract the eye or to serve in any way for protection or mimicry, yet we still find them marked by differences of colour so slight that, unless especially looked for, they would never be noticed.

“Or, again, some species occasionally, but not invariably, show a few white feathers on certain parts of their body, and, when such is the case, it will be found that these white feathers appear on the pœcilomeres. . . . There is hardly a species in which examples of these pœcilomeres may not be found. . . . The Kingfisher (Alcedo ispida) shows the various head pœcilomeres very clearly, and as examples of inconspicuous differences on these tracts, the rump of the hen sparrow (Passer domesticus) and hen chaffinch (Fringilla cœlebs), the malar stripe and dark ear-patch of the hen Yellow Bunting (Emberiza citrinella), and the dark ante-orbital patch of the Barn Owl (Strix flammea) are familiar examples. And, lastly, as an instance of the class where a few white feathers frequently, but not invariably, appear, the young of the cuckoo (Cuculus canorus) forms a good example.

“These spots may, however, appear in a transitory manner, as, for instance, where a change of plumage (not necessarily moult) is occurring.”

As an instance of this, Bonhote cites the case of a young male Shoveler (Spatula clypeata), “in which the metallic colour on the head first showed itself on the post-orbital and auricular pœcilomeres, gradually meeting and joining up across the head with the crown and occipital pœcilomeres, and then finally spreading forwards. And it may be well to note that the joining up of the auricular and post-orbital pœcilomeres formed a metallic patch similar in size and position to that found in the male Teal (Querquedula crecca), and, further, in the last stage, when the whole head, except the portion round the beak, was metallic, the markings are similar to those found permanently in the hen Scaup (Fuligula marila).

“Now, these resemblances taking place in the normal pure-bred wild shoveler, the question of reversion does not come in, and no one would suppose these resemblances due to anything more than transitional variation, and it is the object of this portion of the paper to show that variation in colour follows along definite lines.”