In the below mentioned article, the Darwin’s theory in the origin of species have been summarized as: (1) Overproduction (2) Struggle for Existence (3) Variation (4) Natural Selection or Survival of the Fittest and (5) Heredity and Origin of Species.
And the evidences on which Darwin relied are: (6) The Gradation of Organisms (7) Embryology (8) Comparative Morphology (9) Geographical Distribution (10) Geological Succession of Organisms (11) Changes Under Domestication and (12) The Observed Facts of Mutation.
Origin of Species:
Darwin’s Theory, as explained in the Origin of Species, may be summarized as follows:
1. Overproduction:
Animals and plants in nature give rise to more off springs than could possibly survive. This is also the doctrine of Malthus.
2. Struggle for Existence:
Overproduction causes a struggle as to which of the off-springs would survive. The struggle may be active, as in the case of animals or passive, as in the case of plants. The victors survive and the vanquished become extinct.
3. Variation:
Off-springs do not entirely resemble one another but differ among themselves in details. Such variations sometimes help an individual in the struggle for existence while another variation may be a drawback. Thus, some are better fitted for the struggle while others are ill fitted.
4. Natural Selection or Survival of the Fittest:
It is logical to suppose that the one which is the best fitted for the struggle, ultimately survives eliminating others which are not so well fitted. Thus, in the long run, all the variants that is not well fitted die out leaving only the fittest. Ultimately, it seems as if Nature, acting like a Super-gardener, has exercised some selection during which she has eliminated all the unfit.
5. Heredity and Origin of Species:
When the fit ones survive, the variations that cause their conquest, i.e., the advantageous variations, are passed on to their off-springs. Thus, these variations become a part of their character and they become better species than their parents.
This is how Nature is always evolving newer and newer species which are better than their immediate ancestors and thus causing Progress in the course of Evolution. But, environment may often so change that what was once thought ‘progress’ becomes a drawback. That is why many gigantic plants and animals of the Palaeozoic and the Mesozoic became extinct.
Evidences:
The evidences on which Darwin relied in giving shape to his theory are enumerated as follows by T. H. Huxley, his ablest champion:
6. The Gradation of Organisms:
It is well known how the living plants may be arranged from the simple Thallophytes to the more and more complex types—the Bryophytes, the Pteridophytes, the Gymnosperms and the Angiosperms. A comparative study of the different plant groups easily shows a gradation which can only be explained by Evolution.
Thus, we find:
(a) Evolution in structure and migration to land. From the primitive unicellular plants the multicellular (filamentous, thalloid, and then solid forms of land plants) ones have arisen. These have developed mechanical tissues and a strong conducting system ultimately becoming the higher land plants.
(b) Change in the alternation of generations, evolution of sex and ultimately the seed habit. The type of sexuality with typical flower structure and the alternation of generations that we see in the Angiosperms shows gradation down to lower and lower plant groups. Sexuality and alternation of generations are first established in the algae and then the gametophyte becomes progressively more and more reduced while the sporophyte becomes ascendant. The evolution of the seed habit is the climax attained so far. Similar gradation due to evolution is also seen among the animals.
7. Embryology:
A comparative study of the embryos of different groups of plants shows the great resemblances among these groups which is not clear when the adult plants are considered. The difference between the embryo of a bean and that of a tamarind is not as great as that between a bean plant and a tamarind tree. The embryos of animals are even more characteristic as the embryos of such widely different groups as fishes, frogs, reptiles, birds and mammals resemble one another very strongly.
This led some scientists to state that the embryonic development of animals rapidly recapitulates the stages of evolution of that group (recapitulation theory). Thus, the human as well as the chick embryo show vestigeal gills (Fig. 808) which represent, fishes, i.e., a lower stage of evolution. Acacia moniliformis, in which leaf development has been suppressed by the development of phyllodes, shows normal leaves in the seedling Stage.
8. Comparative Morphology:
On studying the morphology of the same organ in different plant and animal groups one cannot but be impressed by their similarity. One may thus easily trace the evolution of the leaf, the flower, the brain, the heart or the hands and legs.
9. Geographical Distribution:
Allied groups of plants are found to live together. An impassable barrier like an ocean or a high mountain range shows different flora and fauna on either side. It is explained later that isolation has been a very important factor in the origin of new species. These facts point out that related species must have a common origin.
Mention may be made in this connection of the much more recent Age and Area Law of Willis. The more widely distributed area a species occupies the older it is in age. The widely distributed species occupy a more primitive place in the scale of evolution while the more localised species are of comparatively recent origin.
10. Geological Succession of Organisms:
This is one of the most glaring evidences. Older rocks show fossils of simpler and more primitive organisms. As one comes to newer and newer rocks one finds remnants showing the gradual evolution of more complex forms.
11. Changes Under Domestication:
Domestication of plants (e.g., garden plants or crop plants) and animals (e.g., pigeons or dogs) is a very important field where one can closely observe variations in off-springs and the origin of new varieties. Here one sees the processes of nature taking place more quickly by the added selective efforts of men. Thus, thousands of varieties have originated in a comparatively smaller period of time.
12. The Observed Facts of Mutation:
Sometimes individual plants are found, specially among wild species, which greatly vary from the parent species and may form a new species all at once. Darwin called them ‘sports’ but did not lay any special stress on them. But, later on, these variants, called mutants, became as important as the whole theory of Evolution.