CH. VI] ADAPTATION 53 



the plants growing in dry climates (xerophytes) on the other, one 

 began to find characters more or less individual to the species, 

 that had something definite to do with the mode of life of the 

 plants, and which therefore might be called adaptive characters. 

 On the one side one found the somewhat negative characters of 

 absence of strengthening tissue and absence of stomata, with 

 diminution or absence of the roots ; on the other side one found 

 the more positive characters such as sinking of the stomata in 

 pits, hairy or waxy leaves, and in the most extreme cases, such as 

 the cacti, of storage of water in the tissues. But few of all these 

 characters, of whichever group, though they might make great 

 changes in the general look of the plants, were of great importance 

 in the separation of plants into species, or into genera, and still 

 less into families. There is little evidence that even such great 

 adaptations as are involved in the development of hj^drophytes 

 or of xerophytes can cause such great morphological or structural 

 differences as actually exist between plants. A mere glance at the 

 composition of any ecological group of plants that are suited to 

 any given situation is sufficient to show the truth of this. Take, 

 for example, the plants that occur in boggy places in Britain, 

 of which lists may be found in Tansley (44) or Bonnier. There 

 are about twenty genera represented, of which eight are Mono- 

 cotyledons, whereas the average proportion of Monocotyledons 

 is only one in five. Among woodland plants they are one in three, 

 whereas upon cliffs they are absent. Though the differences 

 between them and the Dicotyledons are about the most important 

 structural differences that occur, there is no evidence to show that 

 they have any adaptational value whatever. The bog plants also 

 show both alternate and opposite leaves, superior and inferior 

 ovaries, capsules that are septicidal and loculicidal, and that 

 open by lids, that are divided into several loculi or have only 

 one, whilst there are also achenes, follicles, berries, drupes, and 

 schizocarps among the fruits. The twelve genera of Dicotyledons 

 belong to ten different families, including both Polypetalae and 

 Sympetalae, and so on. In Ericaceae, where two genera occur, 

 one has a berry, the other a capsule. Nowhere is there any indi- 

 cation that the supposed structural adaptation had anything to 

 do with the fact that they all live in bogs, and must therefore be 

 adapted, or suited at any rate, to that mode of life. Other 

 British ecological groups of plants — those of chalk-downs, moun- 

 tains, and dunes, etc. — will show similar results. Everywhere one 

 finds that there are plants showing the important characters of 



