The following points highlight the top five theories of the evolution of organic forms. The theories are: 1. Theory of Eternity of Present Conditions 2. Theory of Special Creation or Creationism 3. Cosmozoic Theory 4. Theory of Catastrophism and Spontaneous Creation 5. Theory of Organic Evolution.
Theories of the Evolution of Organic Forms:
- Theory of Eternity of Present Conditions
- Theory of Special Creation or Creationism
- Cosmozoic Theory
- Theory of Catastrophism and Spontaneous Creation
- Theory of Organic Evolution
Theory # 1. Theory of Eternity of Present Conditions:
This theory suggests that various living forms have existed in this universe without undergoing any transformation or change and will presumably continue to exist unchanged in future also. This old theory does not stand on scientific grounds, hence it has no validity at present.
Theory # 2. Theory of Special Creation or Creationism:
Until the middle of 19th century life was believed to have been created by supernatural power either once or at successive intervals and each species was presumed to have been created separately. The theory of special creation is based on the descriptions of the Bible. It states that the world was made by God in six natural days. On the first day, the basic materials were made out of nothing.
On the second day and third day the ancestors of all the living plants suddenly came into existence. On the fifth day the ancestors of all the living animals were created and finally on the sixth day God created man. This theory was strongly advocated by Father Suerez and John Milton. Linnaeus also supported this theory.
This theory was based on religious faith, hence unscientific and could not be subjected to experimental study.
Theory # 3. Cosmozoic Theory:
Cosmozoic theory was proposed by Richter (1865). According to this theory, the living material (protoplasm) might have been carried accidentally to the earth surface from other planets.
But this idea was discarded on account of the following two reasons:
(i) It does not explain as to how the ultimate source of protoplasm evolved.
(ii) The protoplasmic mass must not have survived in the extreme cold and intensive lethal radiations of interplanetary space during its migration.
Recently in 1961, some structures have been discovered from meteorites (rock particles of stars). They seem to contain fossils of microorganisms, possibly of some algae. This simply suggests the possibility of life on other planets also but still there is no evidence that life could be transported safely from one planet to other through meteorites.
Theory # 4. Theory of Catastrophism and Spontaneous Creation:
Empedocles (5th century B.C.) propounded the theory of spontaneous creation or abiogenesis which states that life has originated repeatedly from non-living materials spontaneously. Anaximander and Anaximenes believed that life appeared from small seeds or ‘spermaia’ which came to earth along with rain drops.
Prominent supporters of spontaneous origin of life were Aristotle, William Harvey, Von Helmont and John Needham.
The following are the refutations of the theory of spontaneous creation:
(i) Francesco Redi (1668) experimentally disapprove this view and showed that the meat if placed under a screen so as to check flies from laying eggs on it never developed maggots spontaneously,
(ii) Lazzaro Spallanzani (1765) demonstrated that even microbes failed to develop spontaneously in a broth, a liquid culture medium, boiled and sealed in ajar. The supporters of this theory argued that heating had spoiled the air in the jars in which spontaneous generation failed to occur. Spallanzani had no answer to this and his experiments thus remained inconclusive.
The theory of catastrophism was propounded by Baron Georges Cuvier (1769 – 1832) in view of the discovery of fossils of certain animals whose existence antedated some of the living types. He believed that the old forms of life disappeared due to some revolution or catastrophes and then some new forms came into existence from time to time. D. Orbringy also postulated that new creations followed catastrophes.
The creations of new types in past correspond to the principal geologic periods.
Theory # 5. Theory of Organic Evolution:
The doctrine of evolution is strictly opposed to the older theory of special and separate creation of each plant and animal.
The morphological similarities found in plants and animals of different periods in the earth’s history, the existence of numerous intermediate forms bridging the gaps between species of one period and those of other, the increasing complexity of structure in both plants and animals through geological periods and the actual demonstrable present day origin of new plants and animals from older types, all support the idea that living organisms on this earth are changeable and they are lineal descendants of pre-existing forms.
New species developed due to modification of pre-existing forms.
In the present century notable workers like H.F. Osborn (1857 – 1935), A.S. Romer, G. Simpson etc. have made valuable contributions in vertebrate palaeontology supporting organic evolution. Among population geneticists notable workers like Haldane, Fisher and Wright have provided mathematical evidence that the natural selection can lead to genetic changes in population.
Finally, the various views on evolution were merged into one broad, coherent theory —– the modem synthetic theory of evolution.
The development of thinking’s regarding evolution started several hundred years ago. Early Greek thinkers like Thales (624 – 548 B.C.), Anaximander (588 – 548 B.C.), Empedocles (495 – 435 B.C) etc., had vague notions about evolutionary processes, because they had no idea about biology. Aristotle believed that the individuals were created by God.
With renaissance, the ideas about evolution quickened and from fourteenth century onward many people have formulated theory of organic evolution in their own way.
Long before renaissance, man had discovered dead remains of plants and animals from the earth. Some of the fossils resembled parts of modem plants and animals and others were quite unlike any known forms. Leonardo da Vinci (1452 – 1519) pointed out for the first time that some of the fossils were undoubtedly remains of animals that had lived in the remote past.
Buffon (1707 – 1788) was the first biologist who discarded the idea of special creation and unchangeableness of animals and plants. He concluded that the animals were plastic and had changed from time to time under the influence of changing environment.
Lamarck (1744 – 1829) proposed for the first time the theory of evolution. He also gave us the first conception of phylogenetic tree or the tree of life.
Charles Darwin (1809 – 1882) was beyond doubt the foremost figure in the history of organic evolution. Since Darwin’s time, the theory of organic evolution has been more and more generally accepted. There is no doubt that it is the only logical way whereby the creation can be interpreted and understood.
The methods in ‘operation’ are, however, not definitely known but we may rest assured that the process has been in accordance with great natural laws, some of which still remain unknown.