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concept of species in zoology

 CONCEPT OF SPECIES It is already been stated that individual organisms which have many features in common and able to interbreed in natural conditions are encompassed by the term species. Species is the smallest basic unit of classification. Its proper characterization is, therefore, of vital importance for a 'Natural' scheme of classification. Identifying species on the basis of reproduction, John Ray (1693) considered all individuals produced by parents of the same types as members of the same species. Linnaeus and Some others distinguished species merely on the basis of structure. Morphological Concept of Species This is the most practical and widely used species concept to describe species since people began to classify organisms. Taxonomists use this concept mostly by default because they know most of the species on Earth only from their morphology. New species are usually named on the basis of morphological species concept; morphological descriptions are all that we need...
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Differences between Arteries and Veins

Arteries 

1- Arteries carry blood away from the heart.
2- These carry oxygenated blood (except pulmonary artery).
3- Arteries walls are thick.
4- These are highly elastic in nature.
5- These do not collapse when empty.
6- These do not have any valves.

Veins

1- Veins carry blood towards the heart.
2- These carry deoxygenated blood (except pulmonary vein).
3- Walls of veins are thin.
4- These are less elastic than arteries.
5- These collapse easily when empty. 
6- These have a series of valves that prevent backflow of blood. 
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