who proposed this aforementioned concepts?
It was used by Linnaeus in early evolutionary theory.
Species
This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. 
when are organisms considered to belong to the same species When any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. Some other ways of defining them include their karyotype, DNA sequence, morphology etc. how are they defined as? A species is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. was there any scientists who contributed to this classification? Some of the early ones include Linnaeus, Ernst Mayr, R. L. Mayden and John Wilkins. who is the second mentioned person? He was an evolutionary biologist,  taxonomist, tropical explorer, ornithologist, philosopher of biology, and historian of science. how did he summarized the darwin's theory? UNANSWERABLE what are morphological species? A group of organisms in which individuals conform to certain fixed properties (a type). The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species.
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Species [SEP] Typological or morphological species
A typological species is a group of organisms in which individuals conform to certain fixed properties (a type), so that even pre-literate people often recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species. This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, different phenotypes are not necessarily different species (e.g. a four-winged "Drosophila" born to a two-winged mother is not a different species). Species named in this manner are called "morphospecies". In the 1970s, Robert R.