Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.

Sentence1: Cnidarians have a hydrostatic skeleton. 
Sentence2: Jellyfish have a hydrostatic skeleton.
hydrostatic

Sentence1: Some birds are pollinators that co-evolved with plants. 
Sentence2: insects pollinate plants.
plants

Sentence1: Speciation is an extension of evolution. 
Sentence2: speciation occurs in response to a change in the environment.

Speciation