When a cow swats the flies from its back, it moves its tail generally from side to side. A whale, however, can't do this. Not that it has to swat flies, but it does have to move its tail up and down in order to swim.
Now while this differing way of moving a tail may sound insignificant, it isn't. It is enough to show that the supposed evolution of whales from land-dwelling mammals cannot possibly be true. Why not? Simply because that little change of direction in the way the tail moves, could not happen without some rather elaborate changes between the side-swinging creatures and those which swing their tails vertically.
Any land-dwelling mammal wishing to evolve into a whale could certainly practise moving its left-right tail in an up-down fashion, and there is no doubt that it could certainly improve up to a point. Maybe even learn to swim faster and catch more fish. But after that its tail movement would begin to crush its reproductive apparatus against its pelvis. This would have a tendency to lower the animal's sexual urges somewhat and it would soon lose interest in reproduction—not a very positive evolutionary step. Taken to extremes, this new tail movement would simply crush the whole pelvis. Such a transition would have no survival value whatsoever. The selective pressures of the environment, or natural selection, would work against any such change of tail on a land-dwelling mammal.
To make the claim as evolutionists do, that land-dwelling mammals evolved into sea-dwelling whales is to claim that there had to be simultaneous accidental genetic changes which allowed the tail to grow larger while the pelvis grew smaller. And all this ignores the problems caused as the ever shrinking pelvis or hip bones reached the point where they were far too small to support the creature's weight on its hind legs, and yet still too large to let the animal move its tail up and down with any efficiency.
Of course, tails are not the only thing on whales that make them different from land-dwelling mammals. To totally convert a land-dwelling mammal into a whale you would also have to replace its sweat glands with thick layers of blubbery fat, change its eyes so that the light rays under sea water are still brought to focus on the retina, change its skin to produce a curious surface efficiently designed to streamline the flow of water, and also find some way to enable it to give birth to young which suckle under water without drowning, a rather essential 'adaptation.'
In other words, if you wanted to make a tail for a whale you could not do it by using evolutionary random chance small mutational accidents on some land-dwelling mammals, no matter how long you let the process take. A whale's tail is too well designed to be made that way. In fact, it shows all the evidence of the intelligent engineering which we associate with deliberate creation.