Mabey one starfish had a problem and grew another leg. Then it gave birth and since its parents had extra legs that starfish will inherit the legs. That is how a lot of changing in species happen.
There are thousands, depending upon how you classify them. The simplest division is Igneous/Plutonic, Sedimentary, and Metamorphic. Beyond this you may classify them as carbonates, sulphates and so...
There is one type of autism. However, there are multiple types of pervasive developmental disorders, of which autism is one.
The pervasive developmental disorders, also known as autism spectrum...