4 
Introduction 
importance to his own particular branch of a subject, but it is impossible to deny the proposition 
that no satisfactory definition of a botanical species can be given without a knowledge of the behaviour 
of the plant under varying conditions of growth, in the wild state and in cultivation, and of the 
extent to which seedlings may vary within the limits of the species. 
It has therefore been my endeavour to obtain plants or seeds of as many species as possible 
direct from the localities from which the types of the species were originally obtained and then to 
observe their behaviour under cultivation. Seedlings of as many species as possible have also been 
raised in order to determine the limits of their variability. In analysing the results thus obtained, it 
has been impossible to avoid feeling how utterly the Mendelian laws have shaken the basis on which 
our ideas as to what constitutes a species were founded. It can no longer be accepted that any Iris 
that breeds true from seed is a species, for Mendelian recessives necessarily breed true when self- 
fertilised. This has been abundantly proved by such instances as those of the white forms of 
/. tectorum, /. sidirica , /. orientalis, etc., among seedlings of which no blue flowered plants have been 
known to appear. And yet these white forms cannot be called species for they only differ from the 
type in the absence of the blue colouring matter. Even such a form as /. versicolor kermesina , with 
flowers that are almost crimson, breeds true to that colour when self-fertilised and so does the form 
of /. pseudacorus in which the brown markings on the falls are absent. Yet neither deserves specific 
rank. 
It is useless to pretend that the task of establishing the boundaries of each species is accomplished, 
for many Iris species are still unknown except as herbarium specimens and of others, for instance of 
the Oncocyclus species, it seems impossible in England to raise seedlings on any adequate scale. On 
the other hand, it seemed better to publish the results already obtained in order that help may be 
invited in dealing with the less known species. It is obvious that it is of very great assistance to 
obtain either living rhizomes or seeds of any wild Iris, especially if accompanied by a note of the 
exact locality in which the plants or seeds were collected and possibly also by some indication of the 
conditions of soil and environment under which they were growing 1 . 
It has in many cases seemed inadvisable to give very accurate measurements of the various 
parts of the different Irises, because the dimensions are liable to very considerable variation under 
varying conditions of position, cultivation, climate, etc. What are really important are not the 
absolute but the relative proportions of the various parts and therefore it would be misleading to 
give definite measurements*. Even seeds are liable to great variation and the drought of 1911 
was enough to reduce by one half the size of the seeds of the tall /. spuria from Kashmir, 
illustrated at Plate XV, although the plants had not been disturbed. It is for this reason that 
little weight need be given to such contentions as that the mere size of the seeds is enough to 
distinguish /. caroliniana Watson as a species from /. versicolor L. Different conditions of soil 
and moisture are quite enough to account for the differences in size given by Watson and he 
does not seem to have insisted on any other real difference. 
It might have seemed desirable to include a calendar showing the dates at which the various 
species may be expected to flower and indeed I had collected data for such a list. Eventually, 
however, further knowledge of the behaviour of species in such widely separated districts as Edinburgh, 
Surrey and H^rault and the extraordinary effects in my garden of the hot spring weather of 1912 
showed that the limits of the flowering season of each species would have to be so widely separated 
as to render any such list as was contemplated of little or no value. 
1 For methods of packing and treatment of freshly imported plants, see p. 16. 
J There are a few exceptions to the rule that the relative measurements of the parts of an Iris do not vary to any 
appreciable extent. One of the most striking examples of such variation is I. subbiflora (see p. 145). 
