There is a basic logic to be used in analyzing evidence of the creation of living beings. We can explain this logic with a simple example.
While walking on barren land, you suddenly find a metal key on the ground. Imagine that you pick up this key without knowing what it's good for and you keep on walking. Again imagine that you come up to an empty house a few hundred metres from where you found the key. And again imagine that you try the key in the lock of the house, thinking it might work.
If the key opens the door of this house easily, what conclusion do you arrive at logically?
It is simple. You conclude that this key belongs to the door of this house. That is, it has been designed specially to open this lock. It is obvious that the same craftsman has manufactured both the lock and the key. Therefore the harmony between them is the product of a conscious design.
Yet, if somebody says to you, "You're wrong. The key you found bears no relation to that lock. It is pure coincidence that that key fits that lock, what do you think?" Of course, you will find this proposition deficient in logic, because in this world there are millions of locks and millions of keys that do not fit. It is obviously impossible for two that fit perfectly, out of millions of different ones, to be located beside each other coincidentally.
Especially if the said key is quite complex with all kinds of ins and outs, that is if it is not straight and simple like a room key. The claim of "coincidence" becomes even more absurd because, each detail on the key must have its counterpart in the lock as well, thus decreasing the probability of this coincidence millions of times.
If there are three locks to the door and you have found not one but three keys lying beside each other and all three keys have each opened one of the locks, would you believe an allegation that these keys are pieces of metal that fit the locks by chance? Furthermore, would you not think that the person who makes such a claim either has mental problems or is trying to deceive you and hide something from you?
The logical result presented by this example is simple but very significant: If there is a one-to-one fit between two independent pieces, that is, all details of these two pieces are in perfect harmony, this proves that there is a deliberate design somewhere. The key fits the lock because it has been consciously made by a skilled craftsman. A video cassette goes into a video machine easily and sits in it perfectly because it has been designed by a purposeful designer.
Looking at all these, the following general solution may be arrived at. If there is harmony between two living beings which is realized by the perfect fit of different organs, we can say that this harmony is clear proof of conscious creation. Since the existing harmony indicates a consciousness that may not be explained by chance and since the source of this consciousness may not be these animals, it is inevitable that we accept the existence of a conscious Creator Who "designs" these animals.