| Tyrannical said: Horse shoe crabs have stayed the same for 400 million years. Is evolution broken or something? |
This is a common sentiment that people specify to discredit evolution. But it doesn't.
(I'm assuming you're right about horse shoe crabs, to convey my point, I know nothing about them 
)
Why have horse shoe crabs not evolved in any noticeable manner for 400 million years?
Because they don't have to! Evolution by natural selection essentially dictates that organisms adapt to their surrounding environment, through genetic processes such as mutations, so as to successfully survive and reproduce from generation to generation.
If horse shoe crabs are perfectly adapted to their environment already, it ensures that these successful genes will be passed down to future generations of horse shoe crabs. If no major ecological or environmental disruption causes them to adapt, then these genes will remain similar until one does (In this case, 400 million years).
