On this page
Your Class 11 biology book defines a species by looks. Then a question asks whether a white-flowered and a red-flowered plant are two species, and looks are exactly what is in doubt. More than three centuries ago an English naturalist named John Ray faced the same problem and answered it with a test you can still use. This post explains his test, in his own Latin, and where it runs out.
Who was John Ray?
The University of California Museum of Paleontology gives his birth as 29 November 1627 in Black Notley, Essex, where his father was a blacksmith. He entered Cambridge University in 1644, became a Fellow in 1649 and was ordained a priest in 1660. A study in Notes and Records of the Royal Society (2000) explains that he left his Cambridge fellowship rather than sign the Act of Uniformity of 1662, and that his first book was a catalogue of plants around Cambridge (1660).
Ray then travelled across England and Europe collecting plants, animals and rocks. He was elected to the new Royal Society in 1667 and died on 17 January 1705. UCMP notes that his 1682 *Methodus Plantarum Nova* classified plants by many features at once (flowers, seeds, fruits, roots) and was the first system to separate flowering plants into monocots and dicots, the split you still learn today.
What problem was Ray trying to solve?
Gardeners in Ray's time kept producing new flower colours and double flowers. If every new look counted as a new species, the list of species would grow every year without end. Ray needed a rule that separated real kinds of plant from passing variations.
He set it out in Book 1, Chapter 20 of his *Historia Plantarum* (1686), titled "De specifica (ut vocant) plantarum differentia", on the specific difference of plants. The book is in Latin. Our translation of the key line reads: "After long and careful searching, no surer sign has occurred to us than distinct propagation from seed."
What exactly does Ray's test say?
Ray spells the rule out in three steps in the same chapter (our translations):
Differences that arise from the seed of the same plant "are accidental, not specific", because they do not reproduce their kind again when sown.
Plants that never arise from the seed of the same species are distinct species.
Species "keep their species forever": one does not arise from the seed of the other, or the other way round.
His example is the carnation. Double-flowered carnations are not a separate species from single-flowered ones, he says, because they grow from the seed of single carnations, and their own seed gives single carnations again. He applies the same logic to animals. Nobody needs more proof that a bull and a cow belong to one species, he writes, than that both are often born of the same parents.
He then lists what does not make a new species: a difference only in flower colour, or in doubled or multiplied petals. He blames such varieties on differences of climate, soil or food.
Worked example: applying Ray's test
Try these the way Ray would, then compare with the modern view.
A purple-flowered pea plant produces seeds, and some of them grow into white-flowered plants. Both forms came from the seed of one plant, so by Ray's test they are one species. Flower colour here is a variety, exactly like his carnations.
Two wild-looking plants grow side by side and look almost identical, but seed from one only ever gives the first kind, and seed from the other only ever gives the second. Ray's test says two species, even though looks alone would have lumped them together.
A horse and a donkey. Neither arises from the other's seed, so Ray's test makes them separate species. Mayr's definition reaches the same answer by a different route, by asking whether horse and donkey populations interbreed and stay reproductively isolated.
Notice the difference in method. Ray asks about descent: what comes from what. Mayr, whose definition from 1942 you can read about in our post on Ernst Mayr and the biological species concept, asks about interbreeding between populations.
What did Ray get wrong?
Ray believed the number of species was fixed. In the same chapter he gives his reason: God rested on the sixth day from creating new species, so new kinds could not be appearing every year. His test was meant to protect that fixed list from gardeners' novelties. Darwin later argued that species do change and split over long times, as our post on Darwin, Wallace and natural selection explains, and Ray's framework could not allow that.
His test also struggles where there is no seed. Ray himself noted that many garden varieties spread not by seed but by cuttings, offsets and runners. Bacteria, fungi that reproduce asexually and fossils are all hard to judge by asking what their seed produces, the same weakness that Mayr's definition has.
How does this connect to your syllabus?
The NCERT Class 11 chapter The Living World (reprint 2026-27) says taxonomic studies "consider a group of individual organisms with fundamental similarities as a species", distinguished by "distinct morphological differences". Ray's test is a reminder that looks can mislead and breeding is the deeper check. For how species are then grouped into genera and families, see our guide to The Living World and taxonomy.
If the difference between a species, a variety and a genus still feels blurry, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise the one idea that is blocking you.
What can a student learn from how Ray worked?
Ray's test came from years of collecting and growing plants himself, not from reading. In *The Wisdom of God* he urged readers not to be merely "book-learned" but to "converse with Nature as well as with books", as quoted by UCMP. He also chose a rule that anyone could check by sowing seeds, rather than one that depended on an expert's eye. A good definition, in any subject, is one you can test.
Try it now: write down two organisms that look different but you think are the same species, and explain in one sentence how Ray's seed test would settle it.
