Ray published systematic works on plants, birds, mammals, fish, and insects, in which he brought order to the chaotic mass of names in use by the naturalists of his time. A basic problem of classification was to decide how much apparent variation can be allowed to plants or animals grouped as a single species. How can one know whether or not two individuals share the “same essence?” Ray’s most influential decision was to define a species as a group of organisms that reproduce the same traits from seed. He wrote in Historia plantarum generalis (1686):
In order that an inventory of plants may be begun and a classification of them correctly established, we must try to discover criteria of some sort for distinguishing what are called “species.” After long and considerable investigation, no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species … Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa.
This was the first recorded biological definition of “species.”
Like Linnaeus (1707-1778) whom he later inspired, Ray searched for the “natural system,” a classification of species that would reflect the “divine order of creation.” Unlike Linnaeus, whose plant classification was based entirely on floral reproductive organs, Ray classified plants by overall morphology; the classification in his book Methodus Plantarum Nova (1682) draws on flowers, seeds, fruits, and roots. In other words, Ray rejected the method of dichotomous division by which species were classified according to a series of pre-conceived, “either/or” criteria, and instead classified plants according to similarities and differences that emerged from observation. As a result, Ray’s plant classification was the first to group flowering plants into what are today known as monocots and dicots. His method produced more “natural” results than “artificial” systems based on one feature alone; it expressed the similarities between species more fully. Ray’s system greatly influenced later botanists such as Jussieu and de Candolle, and systems based on total morphology came to replace systems based on only one feature or organ system. Eventually, Ray’s use of total morphology to classify species would become a powerful tool in the hands of evolutionary biologists trying to infer evolutionary relationships.
A devout Christian of his time, Ray was clearly a creationist. In his book The Wisdom of God Manifested in the Works of Creation (1691), he explained that intricate contrivances like the eye or the hand could not have arisen by chance. Instead, they were designed. And their “perfection” displays the wisdom and benevolence of the Designer. Yet Ray did not define species in terms of special creation, as he explained in a letter to his friend Pfaff:
We imagine that a species is the total descendence [sic] of the first couple created by God, almost as all men are represented as the children of Adam and Eve. What means have we, at this time, to rediscover, the path of this genealogy? It is assuredly not in structural resemblance. There remains in reality only reproduction and I maintain that this is the sole certain and even infallible character for the recognition of the species.
A different aspect of Ray’s work represented another huge advance for science. Whereas many medieval and later theologians had taught that the natural world distracted people from salvation and should be avoided, Ray affirmed powerfully that Nature was a worthy subject for study and reason, and that such activity was pleasing to God. Expounding his belief in “natural theology,” Ray wrote in his Catalogue of Cambridge Plants (1660):
There is for a free man no occupation more worthy and delightful than to contemplate the beauteous works of nature and honour the infinite wisdom and goodness of God.
Also, unlike many academics of his day, Ray cautioned against blind acceptance of authorities; in The Wisdom of God, he wrote:
Let it not suffice to be book-learned, to read what others have written and to take upon trust more falsehood than truth, but let us ourselves examine things as we have opportunity, and converse with Nature as well as with books.
Unlike Linnaeus, who focused almost exclusively on classification for its own sake, Ray began to use classification to address questions in physiology, function, and behavior. He understood that living things showed adaptations to their environments.
- Ernst Mayr, The Growth of Biological Thought: Diversity, Evolution, and Inheritance (Cambridge, MA: Harvard University Press, 1982) pp. 256-7.