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Why did Ernst Haeckel propose the kingdom Protista?

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A diagram showing the hierarchy of animal classification in taxonomy, with colored boxes arranged from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and species. — taxonomy Animals and other organisms are classified within a succession of nested groups that ranges from the general to the particular.

In 1866, German zoologist Ernst Haeckel proposed the kingdom Protista to encompass microorganisms that did not fit neatly into the traditional two-kingdom system of plants and animals.12

Historically, the biotic world was divided into plants and animals based on specific criteria, such as motility, nutrition, and the presence of cell walls. However, as microscopy revealed a vast diversity of microscopic life, scientists struggled to classify these organisms. Haeckel recognized that at the unicellular level, the clear distinctions between plants and animals often broke down, making it impossible to categorize many of these forms as exclusively one or the other. By creating the kingdom Protista, Haeckel provided a taxonomic home for these diverse, primarily unicellular organisms that defied the existing binary classification.12