A critical study of the existing evidence seems to be of great importance in order to ascertain the best way of dealing with new facts, and of estimating the value of the factors concerned. [577] It is manifest that we must be very careful and conservative in dealing with new facts that are brought to our attention, and every effort should be made to bring additional evidence to light. Many vegetable anomalies are so rare that they are met with only by the purest chance, and are then believed to be wholly new. When a white variety of some common plant is met with for the first time we generally assume that it originated on that very spot and only a short time previously. The discovery of a second locality for the same variety at once raises the question as to a common origin in the two instances. Could not the plants of the second locality have arisen from seeds transported from the first? White varieties of many species of blue-bells and gentians are found not rarely, white-flowering plants of heather, both of _Erica Tetralix_ and _Calluna vulgaris_ occur on European heaths; white flowers of _Brunella vulgaris_, _Ononis repens_, _Thymus vulgaris_ and others may be seen in many localities in the habitats of the colored species. Pelories of labiates seem to occur often in Austria, but are rare in Holland; white bilberries (_Vaccinium Myrtillus_) have many known localities throughout Europe, and nearly all the berry-bearing species in the large heath family are recorded as having white varieties. [578] Are we to assume a single origin for all the representatives of such a variety, as we have done customarily for all the representatives of a wild species? Or can the same mutation have been repeated at different times and in distant localities? If a distinct mutation from a given species is once possible, why should it not occur twice or thrice? A variety which seems to be new to us may only appear so, because the spot where it grows had hitherto escaped observation. _Lychnis preslii_ is a smooth variety of _Lychnis diurna_ and was observed for the first time in the year 1842 by Sekera. It grew abundantly in a grove near Munchengratz in southern Hungary. It was accompanied by the ordinary hairy type of the species. Since then it has been observed to be quite constant in the same locality, and some specimens have been collected for me there lately by Dr. Nemec, of Prague. No other native localities of this variety have been discovered, and there can be no doubt that it must have arisen from the ordinary campion near the spot where it still grows. But this change may have taken place some years before the first discovery, or perhaps one or more centuries ago. This could only be known if it could be proved that the locality had been satisfactorily investigated previously, and that the variety had not [579] been met with. Even in this case only something would be discovered about the time of the change, but nothing about its real nature.