V. VARIETIES, ETC. 41 
there is no constant distinction made between species and 
varieties, other than a decision by individual opinion in each 
special instance. Super-species, ver-species, sub-species, heredi- 
tary race or strain, greater variety, lesser variety, variation, are 
simply a series of terms expressing gradations of difference, not 
things shown to be absolutely distinct in their kind. 
No doubt a theoretic distinction may be alleged between them 
at some one of the grades. It is still hypothetically assumed that 
distinct species do exist in nature, and that one real or natural 
Species can never pass into or produce another species. This 
assumption, while accepted as a truth, will occasionally afford a 
practical and seemingly positive test for a variety, although it can 
never become such a test for a species. However dissimilar any 
two plants or sets of plants may be, if it is known that they have 
descended from a common ancestor, or have been produced by the 
same parent,—then they are held to be varieties, or type-form 
and variety, of the species represented by that common ancestor 
or same parent. On the contrary, however similar any two plants 
may be otherwise, if some constant and inconvertible difference 
seemingly exists between them, by which they can be dis- 
tinguished one from the other, one form never having been 
observed to produce or pass into the other form ;—then, any 
botanist may hold himself warranted to name and describe them as 
two distinct species, in reliance on the negative evidence only, and 
with little fear of refutation. 
But here comes the conflict of opinion about species and 
varieties ; because it can never be possible to ascertain that the 
seemingly constant difference is a really inconvertible difference, 
or even that it is a really constant difference. In most cases it is 
made a matter of analogical inference, not one of experimental 
knowledge. A decision must often be given simply in the absence 
of any positive evidence of sameness. Where the differential 
characters do not exceed, either in kind or in degree, those which 
experienced botanists have observed to be convertible between 
other plants, some of them will usually declare the characters 
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