His theory of homology
In his studies, Darwin noticed that different types of creatures shared some common features, such as the five fingers of a human hand and the five digits of a bat’s wing or of a dolphin’s fin. He postulated that this similarity in different species, which he called "homology," was evidence for a common ancestry.
Yet this argument is based on an analogy that’s quite weak since the fossil record shows no gradual evolution of these limbs from one species to another. There is, however, another and simpler way to explain these common features. Instead of having a common ancestor, these similar features could simply be the result of a common design.
We see this common design in how man builds things. We construct a car, a cart and a vacuum cleaner with four wheels, but this doesn’t mean they have a common ancestor —merely a common design. Four wheels happen to give more stability and strength than three wheels and can better distribute the weight on top. We can deduce that a wise designer would have used this type of model of four legs to give stability and strength to many of the creatures that were made, instead of using three legs.
Similarly, the use of five digits in hands, wings and flippers indicates good design features repeatedly used to obtain optimal results. The same can be said for why creatures from frogs to man have two eyes, two ears and four limbs—they are evidence of good design and function.
Really, does it make more sense that a designer used these same patterns because they worked so well, or that blind chance in natural selection and mutations just happened to come up with the optimal design after so many trial-and-error attempts? If the latter was the case, where is the evidence of the many failed models that should have ended up in the scrap heap of the fossil record, as Darwin predicted? No such evidence has been found.
Indeed, when creatures that are supposedly far removed from one another on the evolutionary tree share common advanced characteristics, evolutionists maintain that these characteristics evolved separately. But what are the odds of the same complex characteristic evolving by chance multiple times? Again, common design is clearly a far more logical explanation.