EVOLUTION has come to be regarded to-day as a synonym of progress, but while in the main it signifies a forward movement, the word is equally applicable to retrogressive or degenerative trends. The evolution of living things has been on the whole a process of increasing organisation and efficiency, an advancement from the simplest and lowliest forms to the higher and more complicated. But many striking cases of evolution can be cited in which the movement has been in the reverse direction. As often as not, retrogressive evolution is associated with a change from an independent to a parasitic mode of life. Sometimes this leads to a complete degeneration of the individual, but in other instances this degeneration is accompanied by a specialisation, which is in itself a progressive evolution in so far as the individual becomes remarkably well adapted to fit its new surroundings and mode of life.
Perhaps the best illustration of this is found in the tapeworms. A typical tape-worm has an elongated body, with a head beset with a crown of hooks, enabling it to obtain a fast hold in the tissues of its host. Sense-organs are wanting, and the nervous system is much reduced. The body is covered with a cuticle or outer skin to withstand the action of digestive juices, since most of these parasites are found in the gut of their host, but capable of absorbing the products of digestion elaborated by its host. There is no need for a blood-system, since the sole purpose of this would be to distribute the food material arising from digestion, and in the case of a parasite the whole body is bathed in its food. Excretory organs are, however, present. The remaining organs—those of reproduction—are enlarged out of all proportion to the rest.
In order that the race may survive it is necessary for eggs to be laid and for some at least of the young resulting from
THE HUMBLEST ANIMALS
them to find their way to a new host. In the case of parasites the chances ot this happening are infinitely more remote than those of free-living animals that can choose the season for laying, can select a suitable site to deposit the eggs, can lay by a store of food for the young when they are hatched, or even guard the young until they are able to look after themselves. With parasites it is a much more haphazard affair. This means that a vast number of eggs must be laid in order to ensure that a few shall survive and the young from it find a suitable host.
A tape-worm’s body is divided into segments each of which is filled very largely with the reproductive organs. As the eggs are fertilised and ripen, the segments containing them become detached from the rest of the body and pass to the exterior, usually with the faces of the host. As old segments are thrown off, new segments are budded off from just behind the head. A tape-worm is, therefore, in effect, nothing more than a reproducing machine, living on the energies of another animal and devoting all its time and energy to producing fresh tape-worms to bother other animals of the same species. Truly one of the strangest freaks of evolution imaginable !