Full Idea
It is widely agreed among biologists that no essential property can be found to demarcate species, so that if an essential property is necessary for a natural kind, species are not natural kinds.
Gist of Idea
It seems that species lack essential properties, so they can't be natural kinds
Source
John Dupré (The Disorder of Things [1993], 2)
Book Reference
Dupré,John: 'The Disorder of Things' [Harvard 1995], p.53
A Reaction
This uses 'essential' to mean 'necessary', but I would use 'essential' to mean 'deeply explanatory'. Biological species are, nevertheless, dubious members of an ontological system. Vegetables are the problem.
Related Idea
Idea 17379 Borders between species are much less clear in vegetables than among animals [Dupré]