Why did early scientists believe that plants were fundamentally different than animals?

Ascientist observes certain changes in the physical features of a particular species over time. she concludes that the species has undergone evolutionary change. she wants to study the animals by analyzing their population. she’ll use . the population of this species is decreasing considerably. if this trend doesn’t change, this species will fall under the category of animals. this is a plato question,i need to make up his biology credit.