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It is a system of classification which takes into consideration comparable study of a number of characters so as to bring out natural similarities and dissimilarities and hence natural relationship among the organisms.

The system employs those characters which are relatively constant. They include morphological characters, anatomical characters, cytological characters, physiology, ontogeny or development, reproduction, cyto-chemistry and bio-chemistry, experimental taxonomy and so on.

The characteristics are helpful in bringing out maximum number of similarities in a group and comparable differences with other groups of organisms. For example, mammals are characterised by the presence of mammae, hair, vivipary, 4- chambered heart, dencleated erythrocytes and warm blooded nature.

Birds posses wings, feathers, pneumatic bones, ovipary, 4- chambered heart, nucleated erythrocytes and warm- blooded nature. Similarly, fishes do not posses limbs but fins. Their body is covered with scales. Respiration is through gills. Heart is two- chambered. They are freezing cold- blooded.

In natural system of classification, homology is brought out through the study of internal and external characters. Homology is the relationship of comparable structures having been derived from a common form. For example, the fore arm of different land vertebrates has the same pent dactyl constitution.

It is externally much different in different organisms to perform different functions like grasping in human beings, running in horse, swimming in whale, flight in bird or a bat.

Homology shows how each organ or structure has evolved in different groups to suit different functions. Homology is studied in case of bio-chemicals as well. Molecular homology is the finding of relationship of comparable molecules like DNA, RNA and proteins by studying their similarities and dissimilarities.

Even certain bio chemicals occur in specific groups. For example Betacyanin is found in beet root and related plants. The branch of biology that utilizes the study of chemicals in classification is called chemotaxonomy. Chromosomes or karyotypes are also important foe knowing natural relationship.

In the nineteenth century many comparative studies were not available. Scientists relied more upon morphological and anatomical characters than other characters. A natural system for classification of seed plants was proposed by Bentham and Hooker (1862- 1883) in their three volumes treatise ‘Genera Plantarum’.

These days a natural system of classification not only bring out natural relationships but also studies the evolutionary tendencies and phylogeny with the help of all available data including fossils.

Natural system of classification is certainly better than any artificial system of classification because:

  1. There is stress on actual study of each and every organism.
  2. There is stress on comparative study.
  3. It brings about affinities on the basis of a number of characters.
  4. It brings out natural relationships amongst organisms.
  5. It places only related organisms in a group.
  6. The system prevents coming together of unrelated organisms.
  7. The system indicates phylogenetic relationships and the origin of different taxa.