The Scottish Naturalist. 245 



farmers in agriculture, such as the Serrafalcus group oiBromus^ and 

 Lolium italicum. 



Another source of uncertainty that adds a good deal to the 

 difficulty of deciding on the claims of some species arises from the 

 fact that in the middle ages numerous species of herbs were culti- 

 vated because of their real or supposed medicinal virtues, or as 

 pot herbs, and that a large proportion of these ceased to be culti- 

 vated when other species of greater value or beauty were brought 

 from distant lands. Not a few of the plants formerly cultivated 

 had to some extent naturalised themselves in the gardens, but on 

 falling into disrepute they came to be regarded merely as trouble- 

 some weeds, and their former reputation being lost, the fact that 

 they had once been cultivated was forgotten, and, along with this, 

 their origin fell out of view. Probably more species than we are 

 aware of have been introduced in this way. 



But even among the species in the introduction of which man 

 has had no share, direct or indirect, the claim to be native rests 

 not a little on the length of time since they were introduced, and 

 on their suitability to the new conditions in which they are placed. 

 Probably all floras are receiving slow additions by causes apart 

 from the influences of man ; and it is opposed to all the lessons of 

 nature to endeavour to establish hard and fast laws in this depart- 

 ment of Botany, no less than in others, in which this is already 

 fully recognised. But while not attempting to lay down such hard 

 and fast rules, we may aid in coming to a knowledge of the laws 

 that regulate the changes that the flora of any locality undergoes 

 by noting carefully the first appearance of plants newly introduced 

 into it, whether by man, consciously or unconsciously, or by causes 

 independent of him. 



We may learn something of the laws that enable certain forms 

 to conquer in the struggle for existence, by observing which of the 

 new comers into our plant-world can hold their ground, and the 

 conditions under which they succeed in doing so, and we may be 

 able to detect the causes whereby others, under the most favour- 

 able circumstances, apparently die out and disappear. It is as a 

 contribution in this direction that this list has been prepared. 



An analysis of the list will show a considerable addition to the 

 number of " casuals " recorded in the " Botanist's Guide." A few 

 of these are somewhat striking species, and are not very rare ; but 

 most of them are, as might be supposed, merely sporadic in their 



