I'm posting this, slightly "tongue in cheek" (don't know how this translates from English, but it means something like "not entirely seriously", but also with a sens eof mischief!).
Darwin, in his Origin of Species, said the following:
"Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgment and wide experience seems the only guide to follow. We must, however, in many cases, decide by a majority of naturalists, for few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges."
"Certainly no clear line of demarcation has as yet been drawn between species and sub-species--that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species; or, again, between sub-species and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage. "
"From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience sake."
How much further has botanical science managed to get in the last 150 years? When you consider where physical sciences have got to in that period of time, can anyone explain the difference in progress?