The Theory of Evolution

THE THEORY OF EVOLUTION From the earliest times man must have speculated on the nature and origin of the multitude of living creatures, both plants and animals, which people the surface of the earth. Some have as sumed that the diverse forms with their different shapes and sizes, properties and habits, were each specially created, probably to fill a particular place and serve a special purpose; others preferred to consider them as the gradually developed products of nature.

According to modern doctrine, evolution and the diversity we see around us are due to the action in the past of "natural causes," which can be observed still at work in the present. This concep tion has been applied to the whole cosmos including both living and non-living things ; but in this article we are concerned only with the evolution of living organisms.

Although in the i8th and early i9th centuries many naturalists, notably Erasmus Darwin and Lamarck, maintained that the various forms of life have evolved from one another by some process of transformation, it was not till the middle of the last century that Charles Darwin succeeded in establishing the doctrine of organic evolution on a thoroughly scientific basis. This work of Darwin has been one of the greatest triumphs of modern science. It marks an epoch in human thought, for not only has the doctrine dominated ever since all the sciences dealing with living things, but its effect has penetrated to all regions of scientific enquiry. To secure this success every sort of opposition, religious prejudice and intellectual inertia had to be overcome, and not the least resistance was due to the concept of "species" (q.v.) held by nat uralists themselves.

Species.

This idea of species we owe chiefly to Ray (1628 '705), who first endeavoured to define species as composed of individuals derived from common parents and possessing the same constant well-defined "specific characters." Ray's work was later extended and completed by Linnaeus (1707-78), who founded the modern system of classification and nomenclature. He grouped all known plants and animals into species, genera, families, orders and classes, insisted on the objective reality of species, and intro duced the binomial system whereby each was called by a Latin specific name added to that of the genus to which it belonged. Thus man was designated (genus) Homo (species) sapiens. This was a great practical advance. Organisms could now be more easily described, classified and compared, the floras and faunas of various countries could be contrasted, living could be corn pared with extinct fossil forms. But the adoption of this system was attended by serious dangers. To Linnaeus and his followers the "species" became the unit of creation; it was assumed to have been separately created at the beginning and to have persisted more or less unchanged ever since. All organisms were supposed to belong to species, clear cut, definable; there could be no transi tion between them. Thus from a convenient practical classification Arose a rigid dogma which for long held a dominant position, especially among systematists concerned with the identification and classification of animals and plants. This dogma was accepted by the world at large, and its effects have not yet entirely worn off even in learned scientific treatises. One of the most important results of Darwin's work has been the destruction once and for all of the foundations on which was built the dogma of the immuta bility of species, and indeed the belief in the very existence of such definite units in nature.

It is now universally held by competent biologists that all organ isms, living or extinct, have arisen from remote common ancestors by a process of gradual change or evolution, and further, that living matter or "life" itself, in all probability arose from non living matter in the first stages of this evolutionary process. The only doubt which remains concerns the exact steps in the process, and the nature and relative importance of the various factors which have contributed to it.