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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.

Ch. XI.]

PLAGUE OF TICKS.

209

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 inflaned 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

Ch. XI.]

HAIR AND PARASITES.

211

months and years—often indeed from generation to generation-on the same spot. Monkeys change their sleeping places almost daily. The ourang-outang, that makes a nest of the boughs of trees, is said to construct a fresh one every night. The dwelling-places of savages, often made of, or lined with, the skins of animals, with the dusty earth for a floor, harbour all kinds of insect vermin, and produce and perpetuate skin disease, due to the attacks of minute sarcopti. If the dog by losing its hair should obtain any protection from these and other insect pests, instead of wondering that a hairless breed of dogs has been produced in a tropical country, I am more surprised that haired ones should abound. That they do so must, I think, be owing to man having preferred the haired breeds for their superior beauty and greater variety, and encouraged their multiplication.

CHAPTER XII.

Olama- The “Sanate”—Muy-muy--Idleness of the people-Moun

tain road-The “bull rock”—The bull's-horn thorn-Ants kept as standing armies by some plants— Use of honey-secreting glands -Plant-lice, scale-insects, and leaf-hoppers furnish ants with honey, and in return are protected by the latter-Contest between wasps and ants—Waxy secretions of the homopterous hemiptera.

WE rode up to the large hacienda at Olama, and were asked to alight by a man whom I at first took to be the proprietor, but afterwards discovered to be a traveller like ourselves, buying cattle for the Leon market. The owner of the house and his sister were away at a little town three or four miles distant; and I was a little nervous about the reception we should have when they returned and found us making ourselves at home at their house. Velasquez had, however, no apprehensions on that score, as he knew that throughout the central departments of Nicaragua it is the custom for travellers to expect and to receive a welcome at any house they may arrive at by nightfall. Excepting in the towns, and on some of the main roads, there are no houses where travellers can stop and pay for a night's lodging. Every one expects to be called on at any time to give a night's shelter. This is all that is afforded, as travellers carry with them their hammocks and food. About an

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