« EelmineJätka »
gero's description of it would fit the hairless dog of the present day very well, excepting the size; he says it was four feet long, totally naked, excepting a few stiff hairs on its snout, and ash coloured, spotted with black and tawny.
Tschudi makes two races of indigenous dogs in tropical America. I. The Canis caraibicus (Lesson), without hair, and which does not bark. 2. The Canis ingo (Tschudi), the common hairy dog, which has pointed nose and ears, and barks.* The small eatable dog of the Mexicans was called by them Techichi; and Humboldt derives the name from Tetl, a stone, and says that it means “a dumb dog," but this appears rather a forced derivation. Chichi is Aztec for“ to suck;" and it seems to me more probable that the little dogs they eat, and which are spoken of by the Spaniards as making very tender and delicate food, were the puppies of the Xoloitzcuintli, and that Techichi meant “a sucker.”
Whether the hairless dog was or was not the Techichi of which the Mexicans made such savoury dishes is an open question, but there can be no doubt that the former was found in tropical America by the Spanish conquerors, and that it has survived to the present time, with little or no change. That it should not have intermixed with the common haired variety, and lost its distinctive characters, is very remarkable. It has not been artificially preserved, for instead of being looked on with favour by the Indians, Humboldt states that in Peru, where it is abundant, it is despised and ill-treated. Under such circumstances, the variety can only have
* J. J. von Tschudi, quoted by Humboldt, “ Aspects of Nature,” English edition, vol. i. p. III.
Ch. XI.] NATURAL AND ARTIFICIAL SELECTION.
been preserved through not interbreeding with the common form, either from a dislike to such unions, or by some amount of sterility when they are formed. This is, I think, in favour of the inference that the variety has been produced by natural and not by artificial selection, for diminished fertility is seldom or never acquired between artificial varieties.
Man isolates varieties, and breeds from them, and continuing to separate those that vary in the direction he wishes to follow, a very great difference is, in a comparatively short time, produced. But these artificial varieties, though often more different from each other than some natural species, readily interbreed, and if left to themselves, rapidly revert to a common type. In natural selection there is a great and fundamental difference. The varieties that arise can seldom be separated from the parent form and from other varieties, until they vary also in the elements of reproduction. Thousands of varieties probably revert to the parent type, but if at last one is produced that breeds only with its own form, we can easily see how a new species might be segregated. As long as varieties interbreed together and with the parent form, it does not seem possible that a new species could be formed by natural selection, excepting in cases of geographical isolation. All the individuals might vary in some one direction, but they could not split up into distinct species whilst they occupied the same area and interbred without difficulty. Before a variety can become permanent, it must be either separated from the others, or have acquired some disinclination or inability to interbreed with them. So long as they interbreed together, the possible divergence is kept within narrow limits, but whenever a variety is produced, the individuals of which have a partiality for interbreeding, and some amount of sterility when crossed with the parent form, the tie that bound it to the central stock is loosened, and the foundation is laid for the formation of a new species. Further divergence would be unchecked, or only slightly checked, and the elements of reproduction having begun to vary, would probably continue to diverge from the parent form, for Darwin has shown that any organ in which a species has begun to vary, is liable to further change in the same direction.* Thus one of the best tests of the specific difference of two allied forms living together, is their sterility when crossed, and nearly allied species separated by geographical barriers are more likely to interbreed than those inhabiting the same area. Artificial selection is more rapid in its results, but less stable than that of nature, because the barriers that man raises to prevent intermingling of varieties are temporary and partial, whilst that which nature fixes when sterility arises is permanent and complete.
For these reasons I think that the fact that the hairless dog of tropical America has not interbred with the common form, and regained its hairy coat, is in favour. of the inference that the variety has been produced by natural and not by artificial selection. By this I do not mean that it has arisen as a wild variety, for it is probable that its domestication was an important element amongst the causes that led to its formation, but that it has not been produced by man selecting the individuals to breed from that had the least covering
* See “ Animals and Plants under Domestication,” vol. ii. p. 241.
of hairs. I cannot agree with some eminent naturalists that the loss of a hairy covering would always be disadvantageous. My experience in tropical countries has led me to the conclusion that in such parts at least there is one serious drawback to the advantages of having the skin covered with hair. It affords cover for parasitical insects, which, if the skin were naked, might more easily be got rid of.
No one who has not lived and moved about amongst the bush of the tropics can appreciate what a torment the different parasitical species of acarus or ticks are. On my first journey in Northern Brazil, I had my legs inflanied and ulcerated from the ankles to the knees, from the irritation produced by a minute red tick that is brushed off the low shrubs, and attaches itself to the passer-by. This little insect is called the “ Mocoim” by the Brazilians, and is a great torment. It is so minute that except by careful searching it cannot be perceived, and it causes an intolerable itching. If the skin were thickly covered with hair, it would be next to impossible to get rid of it. Through all tropical America, during the dry season, a brown tick (Ixodes bovis), varying in size from a pin's head to a pea, abounds. In Nicaragua, in April, they are very small, and swarm upon the plains, so that the traveller often gets covered with them. They get upon the tips of the leaves and shoots of low shrubs, and stand with their hind-legs stretched out. Each foot has two hooks or claws, and with these it lays hold of any animal brushing past. All large land animals seem subject to their attacks. I have seen them on snakes and iguanas, on many of the large birds, especially on the curassows. They abound on all the large mammals, and on many of the small ones. Sick and weak animals are particularly infested with them, probably because they have not the strength to rub and pick them off, and they must often hasten, if they do not cause their death. The herdsmen, or“ vacqueros,” keep a ball of soft wax at their houses, which they rub over their skin when they come in from the plains, the small “garrapatos” sticking to it, whilst the larger ones are picked off. How the small ones would be got rid of if the skin had a hairy coat I know not, but the torment of the ticks would certainly be greatly increased.
There are other insect parasites, for the increase and protection of which a hairy coating is even more favourable than it is for the ticks. The Pediculi are specially adapted to live amongst hair, their limbs being constructed for clinging to it. They deposit their nits or eggs amongst it, fastening them securely to the bases of the hairs. Although the pediculi are almost unknown to the middle and upper classes of civilised communities, in consequence of the cleanliness of their persons, clothing, and houses, they abound amongst savage and halfcivilised people. A slight immunity from the attacks of acari and pediculi might in a tropical country more than compensate an animal for the loss of its hairy coat, especially in the case of the domesticated dog, which finds shelter with its master, has not to seek for its food at night, and is protected from the attacks of stronger animals. In the huts of savages dogs are greatly exposed to the attacks of parasitical insects, for vermin generally abound in such localities. Man is the only species amongst the higher primates that lives for