Between living beings and the environment in which they live there is a fit, a fundamental harmony for survival.
The pink flamingo, for example, lowers its head to the swampy ground in which it lives to fetch its food there; The hummingbirds, with their long beaks, are adapted to collect nectar from the tubular flowers they visit.
The adaptation of living beings to the environment is an undeniable fact. The origin of adaptation, however, has always been discussed.
In ancient times, the idea that species would be fixed and unchanging was defended by the Greek philosophers. The so-called fixists they proposed that living species had existed since the planet's origin, and the extinction of many of them was due to special events such as catastrophes that would have wiped out entire groups of living things. The Greek philosopher Aristotle, a great scholar of nature, did not admit the occurrence of transformation of the species. He believed that organisms were distributed on a scale that ranged from the simplest to the most complex. Each living being on this scale had its definite place. This Aristotelian view, which lasted for about 2,000 years, admitted that the species were fixed and unchanging.
Slowly, from the nineteenth century, a number of thinkers came to accept the idea of the gradual replacement of species by others, through adaptations to continually changing environments. This current of thought, transformist, explained adaptation as a dynamic process, contrary to what the fixists proposed. For transformism, adaptation is achieved through change: as the environment changes, the species changes. Those adapted to the changing environment survive. This idea gave rise to the evolutionism.
Biological evolution is the adaptation of species to continually changing environments.. Adaptation does not always imply improvement. It often leads to simplification. This is the case, for example, with tapeworms, flat parasitic worms: having no digestive tract, they are perfectly adapted to parasitism in the digestive tract of man and other vertebrates.