Plagiarism

Plagiarism, this was an exciting conversation in visual advocacy. It was interesting to hear the views of others on the subject. Looking back in history, artists such as Matisse, Gauguin, and many others took inspiration from other cultures such as Oceania, Africa, Japan, and many more. Many of the paintings we study today, has a clear influence of these inspirations. Some of them are not changed much, is this considered plagiarism, I am curious. I am curious because in our discussion in class, it was said that inspiration like this is plagiarism in some opinions.
I have noticed how we as a team of students inspire each other and ones work influences another's. Is this plagiarism? Ones idea develops another students idea within their own work or dp project, should one be offended or flattered? Is it plagiarism or is it collaborative inspiration?