"Lead cat-and-dog life" - the known expression. So speak about people who constantly quarrel in an occasion and without. The thought of hostility of cats and dogs is so habitual that people often do not even think why so occurs.
On science...
Historically cats were never production of dogs. Therefore here it is impossible to draw parallels with a cat and a mouse. Nevertheless for some reason from time immemorial these animals cannot make friends in any way.Scientists have several versions in this respect. Generally they are constructed on difference in "languages", characters, types of behavior of these animals. It distinctions are noticeable even in obvious: the cat wags a tail when she is angry, and the dog – when is happy. Both cats, and a dog by the nature hunters. But hunt absolutely differently. Dogs are hardy and got used to exhaust the victim, running huge distances. Cats are not able to move so quickly and are not hardy at all, but are able to creep quietly to the victim. Therefore cats have no specific smell, and dogs is expressed smell of a dog. Other version of scientists is a genetic competition of types. It is quite possible that saber-toothed tigers who were capable to offend ancestors of domestic dogs were ancestors of cats. Scientists consider that the current hostility of dogs to cats is caused by the desire put at the genetic level to revenge descendants of the enemies. One more argument – cats and dogs just did not get used to each other. Not everything, but in the majority the people accurately are divided into "dog lovers" and "aelurophiles". That the few who decide to get both a cat, and a dog often are perplexed – why they are considered enemies? The kitten and a puppy living with the early childhood nearby almost always become the best friends. But the fact is that these animals get used to each other. And the dog who did not get used to a cat has an insuperable desire to pursue this noisy fluffy lump whereas the cat is frightened attacks, begins to be protected and run away. Besides, to a cat own territory is important. For this reason cats often mark places which consider own. Any invasion of other animal, including dog, into this place is perceived as a signal to action. The dog responds to any attack. As on the speed and force the dog wins against a cat more often, for the last it in most cases comes to an end is deplorable.
... and by hearsay
Of course, for many years the hostility of a dog and cat acquired a set of legends and beliefs. Perhaps, the most beautiful legend – about the poor and a magic ring. Nothing was impossible to very poor person – there was no harvest, in the river the fish was not caught, everything fell from hands. But once it found a usual ring in the forest – and life got better after that. The cat thought that this ring – magic, and exactly thanks to it affairs went uphill. The cat blabbed out it to the rich man who came to the house and took away a ring from the poor. Wishing to help the owner, the cat and a dog stole a ring at the rich man. But on the way home the dog fell asleep. The cat returned a ring to the owner, having told that she got it, and the dog told the rich man about a ring. Then the owner expelled a dog to the yard and began to feed with scraps, and left a cat in the house and fed from a bowl. Such treachery the dog did not forgive to a cat.