Chances of Life 285 



selves, and, if there are not the right insects to do it 

 for them, they cannot produce any seed. Such plants 

 as have more than one ' form ' (see p. 222), and such as 

 bear pollen and ovules on separate individuals, run, of 

 course, especial risks when they migrate, as one is 

 generally helpless without the other. 



In the matter of soil, too, a very short distance often 

 makes a great difference. The bee-orchis for example, 

 grows plentifully on the chalk in Surrey, and one year 

 it suddenly made its appearance in a clayey field near 

 Thames Ditton, where it had never been seen before. 

 About a hundred plants blossomed in this one field, 

 and nowhere else in the neighbourhood. But they 

 gradually dwindled away, and in the course of six or 

 eight years all had entirely disappeared. The seeds 

 had probably travelled to the clay-field in some chalk 

 which had been brought from another part of the 

 county, but the colony could not be a lasting one for 

 some reason most likely owing to the change of soil, 

 and perhaps also of situation. 



Then, again, there is the milk-thistle, which has run 

 riot in Australia, but has utterly failed to make a settle- 

 ment in New Zealand. Single specimens have been 

 grown in the latter country, and have seeded freely. 

 The seeds, too, after the manner of their kind, must 

 have been carried hither and thither by the wind in 

 multitudes ; but .still the plant has not spread, and is a 

 failure a fortunate failure as an emigrant. 



Other European plants, to the number of more than 

 a hundred and fifty, have been quite successful in New 

 Zealand, and are thoroughly established ; but, it is re- 

 markable that, while so many European plants have 

 made themselves at home there, only two or three 



