THE THEORY OF EVOLUTION

LINNAEUS, who established the natural system of classification, firmly believed in the doctrine of Special Creation, which stated that each species of plant and animal was separately created and that all the plants and animals alive to-day are descendants of the originally created ones and resemble their ancestors in all but minor points, I.e. species are unchangeable. The opposed idea, which the majority of biologists hold to-day, is that of Evolution which implies that during past ages, animals and plants have changed in form, and the changes which have occurred have resulted in the repeated production of new species from previously existing ones. For example, all the various species of the genus Canis are supposed to have originated from a common ancestral form by a series of changes during past generations. On this basis the resemblances shown by the different species of the genus Canis are due to a natural relationship by descent from a common ancestor, just as certain family resemblances among cousins may be traced back to a common grandparent. Once this idea is acoepted, the different genera in a particular family of animals or plants are regarded as also being descended from a common ancestral type farther back. Carrying this to its logical conclusion, the whole of the animal kingdom is supposed to have arisen from simple unicellular types similar to Amoeba, whose descendants became progressively more and more complex. Just as in an actual tree some of the lower branches die, so it is supposed various types of animals evolved in the past may have become extinct, and this is borne out by fossil evidence. In the same manner a tree of descent may be traced out for the plant kingdom.

Tree of Descent of Animals.

Again, such a tree is largely conjectural, but is borne out by the existence of the various plant types which by comparison can be seen to form a series of increasing complexity, but with each type indicating an advance on the one preceding it. Thus in the land plants from the mosses upwards, the gametophyte generation is seen declining in structure and importance, while the sporophyte generation becomes more and more advanced and dominant as one studies the ferns, club-mosses, gymnosperms and flowering plants. In fact we can trace a series of plants showing progressive adaptation to a life on land, the gametophyte being gradually suppressed while the sporophyte, which is better adapted for life on land, becomes more and more dominant in the life-cycle.

In animals, too, we can see progressive adaptation to life on land beginning with the amphibia and proceeding through the reptiles to the birds and mammals. The necessity for water for fertilization is overcome by the method of internal fertilization in reptiles, birds and mammals, while the developing egg is sheltered either in a shell or in the womb of the female, as in the higher mammals. Moreover the adoption of the warm-blooded nature in birds and mammals has enabled these animals to endure more sucoessfully the wider variations in temperature which may occur on land as compared with water, and so to dispense with the necessity for hibernation in most cases.

Evolutionary change and adaptation to new environments have frequently gone hand-in-hand. This is most clearly seen in the mammals and insects. Such evolution is called Adaptive Radiation.

Evidences for Evolution.

Fossil Evidence

Many fossilized remains of plants and animals have been found in the various geological strata of the sedimentary rocks. The older strata contain fossils of simpler types than those found in the later ones. Moreover among the fossils are found those of extinct types, and some of these appear to be intermediate forms forming links between big groups. For example, an ancestral extinct type of bird called Archasopteryx has been found in fossil form which, though possessing feathers like modern birds, had a skull and teeth like those of a reptile while its wings possessed more digits than those of the birds. Birds have certain features similar to those of the reptiles, such as the scales on their legs, and Archasopteryx may represent a stage in the evolution of birds from reptiles.

A series of fossils has also been found showing how the modern horse may have evolved from a small mammal with five digits on its feet through a series of descendants in which the digits were progressively reduced to one accompanied by an increase in size of the whole animal.

Evidence from Similar Basic Structure

The limbs of the land-living vertebrates are all built on the pentadactyl plan, but in birds, bats and the extinct flying reptiles the wings are modified versions of this type of limb and adapted for flight. The fore-limbs of the whales are also pentadactyl but altered in shape to form paddles used in swimming. The most obvious conclusion is that these vertebrates were all descended from terrestrial forms with the usual pentadactyl type of limb, which underwent modification to suit the changed habit.

Evidence from Embryonic Development

The life-history of the frog could possibly be explained by assuming that the ancestors of the frog were originally fish, which later developed lungs and pentadactyl limbs and came to live on land. Now each generation of frogs repeats, as it were, the history of the evolution of the race of frogs, and during development from the egg passes through stages similar to those passed through during the evolution of the modern frog from its ancestral fish-like form. Actually there are fish called lung-fish which live in rivers as far apart as the Amazon, the rivers of Nigeria and certain rivers in Australia. These possess lungs as well as gills which they use in the dry season, which they pass through by burying themselves in the mud. Moreover their eggs develop into tadpoles with external gills which later metamorphose into the adult fish, which suggests that the frogs evolved from fish of this type. The embryos of the higher land vertebrates all have at some stage in their earlier development ’gill-slits ’which are not used and so soon close up. These suggest that the remote ancestors of the higher land vertebrates may have been fish.

Evidence from Vestigial Structures

In the higher vertebrates there occur structures which are no longer used but which have useful counterparts in the lower types. Reptiles and birds have a third eyelid which can be drawn transversely across the eye, but in mammals, including man, this is represented by a triangular vestige in the inner corner of the eye. That of the cat can still be partly drawn across. Other examples are the appendix in carnivorous mammals and in man and monkeys, which is small and useless, whereas in certain herbivorous mammals, e.g. rabbit and horse it is well developed and used for the bacterial digestion of cellulose. The muscles moving the ear are ill-developed and useless in man and monkeys but are used in horses, cats, dogs and many other mammals to move the external ear to catch sound waves. It may be supposed that the original ancestral mammals possessed these structures in a well-developed form but that, during the course of evolution, they may have become no longer needed and so have degenerated.

Present-day Appearances of New Forms

In all species variation occurs. No two individuals of any species are exactly alike, but what is more striking is the sudden appearance of new types called mutations. Among wild rabbits, which are a yellowish-grey colour, black individuals are occasionally found. Other cases of a similar nature occur. They do not always survive in a state of nature, but when noticed by man among his domestic animals and cultivated crops, they may be preserved by him and used to start a new race or breed. In this way a considerable number of new breeds have been established in dogs, cats, fowls, cattle, pigs and sheep, while among cultivated plants many new varieties have been raised from the original wild type, as shown by the existence of many varieties of apples and other fruit trees, and of flowers such as sweet peas. The coming into existence of these new types would tend to support the idea that species of animals can change contrary to the doctrine of the immutability of species originally produced by special creation.

Enough has been said to show what is meant by ’evolution, ’ but the way in which it has occurred is a subject of much debate among biologists. A number of theories as to the causes of evolution and of the ways in which it may have occurred have been put forward. One of the earliest was that of Lamarck, who ascribed evolution to change of habit resulting in changes in the structure of organs to fit the new habit or to a change in the environment causing adaptations of structure. He supposed the long neck of the giraffe to have been developed owing to the giraffe changing its habit of eating grass to feeding on tree foliage which necessitated its longer neck to reach the leaves. Darwin, on the other hand, thought that the struggle for existence or a change in environment operated to favour any varieties in a species which were better adapted for this struggle or change in environment, while the weaker and less adapted ones died out. This process he called Natural Selection and he considered that it was the main cause of the evolution of new species from previously existing ones. All one can usefully say at this stage is that no theory has yet been produced which is regarded by all biologists as being a completely satisfactory explanation of the causes of evolution. The origin of life and the process of creation remain a profound mystery.

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