Why is On the Origin of Species described as "one long argument"?
It uses a lot of scientific terms.
It is based on a theory
It presents an opinion supported by reasons.
It is about evolution

The correct answer is It uses a lot of scientific terms.

The book On the Origin of Species, a seminal scientific work by the British naturalist Charles Darwin, is published in England on November 24, 1859. Darwin's theory argued that living organisms evolve through a process he called "natural selection". In it, organisms with genetic variations that adapt to their environment tend to propagate more offspring than organisms of the same species that lack the variations, thus influencing the genetic structure in general of the species.

Darwin, who was influenced by the work of French naturalist Jean-Baptiste de Lamarck and English economist Thomas Malthus, found much of the evidence in favor of his theory during the long research expedition aboard the HMS Beagle in the 1830s, which lasted close five years. Visiting different places like the Galapagos Islands and New Zealand, Darwin acquired a very close knowledge of the flora, fauna and geology of many lands. His studies on variation and crossing after returning to England, are incompatible with the information collected and with the development of his theory of organic evolution.

The idea of organic evolution was not new. It had been suggested before, among others, by Darwin's grandfather, Erasmus Darwin, a distinguished English scientist and by Lamarck, who in the early 19th century drew the first evolutionary diagram - a chain that led from single-celled organisms to man. However, it was only with Darwin that science presented a practical explanation of the phenomenon of evolution.

i think its provide for the common defense

definitely a for sure. ill see and update if i get the answer