Yeah but cross breeding isn't going to put fish genes into a tomato. The only reason things can cross breed is because they are of the same species.
The definition of species is really blurry when it comes to how different two individuals need to be to be considered different species.
For example, there is a ring of different newt populations in North America, I believe there are 6 or 8 populations. Because of their breeding habits, group 1 is the same species as group 2, which is the same species as group 3, which is the same as 4, the same as 5, the same as 6, however, group 6 and group 1 are considered different species.
Ancient cultures have long been taking different species and crossing them either via breeding, of grafting.