In my opinion, you need to have more of a difference between evolutions than just getting bigger and more flames. I almost thought that the second and third stage were the same sprite in two different poses. D:
And your lines could use some adjusting and be a tad bit cleaner. Some of them are too straight for an organic thing, and need to be curvier.
I also think your shading could use less contrast between the highlights and middle shades, and maybe a little more contrast between the middle shade and the shadows... mainly in the fire starter line.
Iunno, those are what I thought when I looked at your sprites.
i know i need it changed. i was trying to get the fire line to go:
puppy>dog>wolf>werewolf
but the dog and wolf look too similar. the grass goes
leaf>branch>tree>???(need idea!!!)
water goes
tadpole>whatever it is called inbetween a tadpole and frog>frog>???(need idea!!!)
UPDATE:
well, while the internet was down because of a tornado, i made a crappy version of the 3rd grass evolution.

grass/fighting
i know it sucks, but i still want constructive critizism