104 TEST CASES [ch. xi 



actual fact, however, there is rarely much or any structural like- 

 ness among the members of a given association of plants, unless 

 they happen to belong to the extremes of the principal ecological 

 divisions like xerophytes on the one side and hydrophytes on the 

 other, or to special ecological groups like climbers or parasites, 

 which do not, incidentally, grow in any special conditions, or in 

 associations. Even in these cases, the ecological characters that 

 mark them are rarely such as have great importance in classi- 

 fication. 



Were it not for the great structural differences that exist, we 

 could not tell that evolution had gone on to so great and complex 

 a degree. There might be herbs, shrubs, and trees, water-plants, 

 epiphytes, climbers, plants of dry climates, bulbs, tubers, and so 

 on, with other more or less adaptive forms, but there seems no 

 a priori reason to suppose that we should find such things pro- 

 duced by an adaptive evolution as the structural differences that 

 mark whole families like the grasses or crucifers, and distinguish 

 them from one another. As Went has said (50), we see the mor- 

 phological differences, and assume that they must have some 

 physiological explanation. But there is nothing to show that 

 there is any physiological need for them. What connection can 

 be shown between the great bulk of the structural features of 

 plants and their physiological necessities? Man is adapted, region 

 by region, to almost every kind of conditions that can be found 

 upon the surface of the earth, yet he is all undoubtedly of one 

 species, and does not show any great structural differences. And 

 there are numerous similar cases with plants, though these are 

 slower in movement, and have not covered so much ground. 

 Some cover a very large area with no serious structural dif- 

 ferences, like Hydrocotyle asiatica, Sanicula europea or Hippuris 

 vulgaris, while in other places where the conditions are very much 

 alike throughout, a genus may show a number of species. One 

 can rarely infer from the external features of a plant, e.g. in a 

 herbarium specimen, or even in a living one, from what kind of 

 conditions it came. In the vast majority of cases, the most 

 minute morphological description will convey nothing as to the 

 habitat or the physiology, unless the plant happens to belong to 

 one of the great ecological groups like water-plants or climbers. 

 Can anyone read the characters in the most minutely descriptive 

 flora, and locate the probable types of habitat of the plants? 



Taking genera with more than one species in the British flora, 

 the first, Thalictrum, the meadow-rue, has three. T. alpimim, 



