Democratic Sentinel, Volume 10, Number 31, Rensselaer, Jasper County, 3 September 1886 — Change of Structure Due to Change of Function. [ARTICLE]

Change of Structure Due to Change of Function.

* * * Here, however, my chief purpose is to add an instance showing, even more clearly, the connexion between change of function and change of structure. This instance,'allied in nature to the other, is presented Dy those varieties, or rather sub-varieties, of dogs, which, having been household pets, and habitually fed on soft food, nave not been called on to use their jaws in tearing and crunching, and have been but rarely allowed to use them in catching prey and in fighting. No inference can be drawn from the sizes of the jaws themselves, which, in these dogs, have probably been shortened mainly by selection. To get direct proof of the decrease of the muscles concerned in closing the jaws or biting, would require a series of observations very difficult to make. But it is not difficult to get indirect proof of this decrease by looking at the bony structures with which these muscles are connected. Examination of the skulls of sundry indoor dogs contained iy the Museum of the College of Surgeons, proves the relative smallness of such parts. The only pug-dog’s skull is that of an individual not perfectly adult; and though its traits are quite to the point they cannot with safety be taken as evidence. The skull of a toy-terrier has much restricted areas of insertion for the temporal muscles; has weak zygomatic arches; and has extremely small attachments for the masseter muscles. Still more significant is the evidence furnished by the skull of a King Charles’s spaniel, which, if we allow three years to a generation, and bear in mind that the variety must have existed before Charles the Second’s reign, we may assume belongs to something approaching to the hundredth generation of these household pets. The relative breadth between the outer surfaces of the zygomatic arches is conspicuously small; the narrowness of the temporal fossae is also striking; the zygomata are very slender; the temporal muscles have left no marks whatever, either by limiting lines or by the character of the surfaces covered; and the places of attachment for the masseter muscles are very feebly developed. At the Museum of Natural History, among skulls of dogs there is one which, though unnamed, is shown by its small size and by its teeth, to have belonged to one variety or other of lap-dogs, and which has the same traits in an equal degree with the skull just described. Here, then, we have two if not three kinds of dogs which, similarly leading protected and pampered lives, show that in the course of generations the parts concerned in clenching the jaws have dwindled.— Herbert Spencer, in Popular Science Monthly for April.