1.a.1 – What do we mean with species?
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What do we mean with species?
The term species is synonymous with variety, gender, type, but in biology has taken a precise and technical meaning of fundamental importance to become one of the basic concepts of modern biology.
Men see living around them so many varieties of animals and it can be observed that these animals generate cubs of their own kind by mating with their own kind; horses generate other horses, dogs other dogs, etc.., just as men generate other men. The plants, animals and men who die are therefore replaced by the new born, generation after generation, in an endless cycle that, like the cycle of seasons, is repeated always equal to itself, forming an extremely stable picture.
Every form of life appears therefore as a manifestation of the many and immutable laws of nature, which form a smooth and reassuring context; bear with patience every winter knowing that then inevitably spring comes, as after the night a new day will come, after a thunderstorm, the calm will return etc.. and the repetition of the events thus infuses security to the extent that allows to provide that the hardships sooner or later certainly will end and good things will return.
Likewise, you can observe that mixed mating (between different kinds) will not take place or do not generate children; anyway, considering the domestic varieties, sometimes this happens: If we look at the many breeds of dogs, we can in fact note how these have the opportunity to cross; two groups of dogs of different races, placed in the same pen, form mixed couples that generate puppies with mixed physical characteristics. Once grown, these puppies also form couples, mating both among themselves and with the original groups and, after several generations, the peculiar characters of a race will also present in the other descendants and vice versa, there will be no more two groups distinguishable for their appearance but one group born from the merger of the first two; by farming chickens and pigeons instead, this won’t happen. The same phenomenon happens even in a forest or in another natural environment, in the presence of different varieties with the possibility of intercrosses in reproduction.
If we call “races” the varieties with the possibility of intercrosses and we call the other “species”, we can therefore say that in the same environment cannot coexist for long time two or more different races because the same tend to combine giving rise to a single genre. In a single natural environment therefore we can only find different species, which differ from the races for their inability to interbreed, regardless of the appearance of their physical characteristics; this is a general rule by definition, valid for all species of animals we see around us; they are separated from the point of view of reproduction and are kept separated by nature and therefore it is not just a question of appearance or a simple human convention.
The stable repetitiveness of species, both for animals and plants, is possible thanks to a reproductive isolation, which is the reason why biology bases the concept of species on the reproductive capacity. Giving therefore the name “horse” to a given reference animal, will also be horses all the animals that can inbreed with it (or who appear like it, even though of the same sex) to generate other horses, able in turn to repeat the cycle of reproduction. Are excluded all other animals, including donkeys that can generate with horses only sterile offspring.
Still using the example of the many breeds of dogs, it is often possible to find some of them with enormous physical differences, but to continue on existing in a distinct manner, they must be kept separate, otherwise they would intercross. The existence in nature of different races of the same species is therefore due to the fact that these races live in different regions, separated by a long distance, by a mountain range or by the sea, as well as domestic races are kept separate in different farms or by a simple fence.
Since the concept of species is linked to stability, immutability in succession of generations, one can understand the difficulty, even in the absence of religious dogmas, to accept the idea that the various forms of life can change over time.
ROYAL BOX
CARL VON LINNE’ 
Well, the human race is definitely the same, now, as it used to be 1,000 or 10,000 years ago: same problems, same behaviours. But even 10,000 years is only a breath of the Earth’s history. We often miss the general scenario.