Homology
Homo=same
logy=combing of (I think)
so it basically says that you can point to a common ancestor because of a similar design (e.g. teeth, stomach, etc.) found in almost all creatures.
This is not a valid argument as just because something is similar does not mean it originated from the same source.
For example,
an Audi car and a Ford car may have similar features, like an engine, seatbelts, steering wheel. However, just because they are similar does not mean the same engineer designed both cars.