Philosophers of science such as Popper and Kitcher say that it is. Scientists such as Mayr, Dobzhansky, and Ridley agree. Many organizations have passed resolutions to this effect. However, the important question is whether these authorities can back up what they say with evidence.
The following list gives a few of the predictions that have been made from the Theory of Evolution:
For example, arthropods all have chitinous exoskeleton, hemocoel, and jointed legs. Insects have all these plus head-thorax-abdomen body plan and 6 legs. Flies have all that plus two wings and halteres. Calypterate flies have all that plus a certain style of antennae, wing veins, and sutures on the face and back. You will never find the distinguishing features of calypterate flies on a non-fly, much less on a non-insect or non-arthropod.
Dogs are another example. There should be species we would group with dogs, and there are - such as wolves and coyotes. So we are not surprised when dogs and foxes turn out to share some peculiar features of the middle ear. This group - the Family Canidae - can be grouped with the bears, raccoons and weasels, because their ears have some similarities to those of dogs. All of these have carnassial teeth, but so do cats, civets and seals - so we group the entire lot as being Order Carnivora. Carnivores all have 3 middle ear bones, mammary glands, placental development, hair, a diaphragm, a four-chambered heart, and a larynx. But they share those features with humans, bats, elephants and whales. So we group that entire lot as being Class Mammalia. But mammals have amniote eggs, and so do birds, lizards, snakes and turtles. And amniote animals share with frogs and salamanders the property of having four legs - they're tetrapods. Tetrapods and fish both have backbones - they're vertebrates. Vertebrates and starfish are both deuterostomes because they share the way their embryos develop a mouth. Deuterostomes are left-right symmetric, so we group them and insects and snails as bilateral. The bilaterals, the jellyfish and sponges are all animals. Animals, fungi, rose bushes and amoebas all have a nucleus inside each cell - they're eukaryotes. Eukaryotes and bacteria and archaea share the DNA mechanism, lipid-based cell membranes, and hundreds of other biochemical details.
(And that's the short version of the story! For all the fancy Latin names, see the Tree of Life.)
Notice that the dog-to-bacteria story has some apparent irregularities. For example, I said that elephants and whales are mammals, and that mammals have hair. It is not obvious, but elephants and whales do have a small amount of hair. Also, scientists group whales and snakes as tetrapods. So where are their four legs? From the theory of Common Descent, we see that they must be descended from four-legged creatures, and that they have lost their legs. (Loss is an easy mutation - as witness hairless dogs.) So, we predict that there should be fossils of whales with legs, and snakes with legs. These fossils have been found. Similarly, starfish outwardly have radial symmetry, but we classified them as bilateral. So Common Descent predicts that their group (echinoderms) had bilateral ancestors, and such a fossil has been found.
"If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection."This challenge has not been met. In the ensuing 140 years, no such thing has been found. Plants give away nectar and fruit, but they get something in return. Taking care of other members of one's own species (kin selection) doesn't count, so ants and bees (and mammalian milk) don't count.
Chapter VI, Difficulties Of The Theory
The creature that pollinated that orchid was not learned until 1902, forty years later. It was indeed a moth, and it had a 25 cm tongue. And in 1988 it was proven that moth-pollinated short-spurred orchids did set less seed than long ones.
This has been done for pancreatic ribonuclease in ruminants. (Cows, sheep, goats, deer and giraffes are ruminants.) Measurements were made on various ruminants. An ancestral sequence was computed, and protein molecules with that sequence were manufactured. When sequences are chosen at random, we usually wind up with a useless goo. However, the manufactured molecules were biologically active substances. Furthermore, they did exactly what a pancreatic ribonuclease is supposed to do - namely, digest ribonucleic acids.
Therefore, it should be possible to analyze an aquatic creature's bones, and tell whether it grew up in fresh water or in the ocean. This has been done, and it worked. We can distinguish the bones of river dolphins from the bones of killer whales.
Now for the prediction. We have fossils of various early whales. Since whales are mammals, evolution predicts that they evolved from land animals. And, the very earliest of those whales would have lived in fresh water, while they were evolving their aquatic skills. (Skills such as the ability to do without fresh water.) Therefore, the oxygen isotope ratios in their fossils should be like the isotope ratios in modern river dolphins.
It's been measured, and the prediction was correct. The two oldest species in the fossil record - Pakicetus and Ambulocetus - lived in fresh water. Rodhocetus, Basilosaurus and the others all lived in salt water.