178 SCIENCE PROGRESS. 



leaves, which fold over each other like the leaves of a 

 book. A further example is also seen in the Utricularias. 

 The only forms which occur in Europe are aquatic plants, 

 and which have lone attracted notice on account of their 

 bladders which serve as traps for small animals. But the 

 biology of the whole plant, and the meaning of its separate 

 parts, was not evident till it became possible to examine 

 the mode of life of their numerous congeners. 1 Many of 

 these are terrestrial species, and it appeared — incredible as 

 it may perhaps seem to many — that the Utricularia plant 

 — as represented in the European forms — corresponds to 

 one leaf. But if any one doubts that such a far-reaching 

 change in the development of a leaf, an adaptation of 

 functions so widely divergent from the general rule, can 

 possibly occur, the explanation of his position is to be sought 

 in the relatively commonplace character of our native flora. 

 If, on the contrary, we look at the tropical plants, we find 

 among them some whose leaves penetrate into the ground, 

 and function as roots, serving as organs to keep the plant 

 firmly attached to the substratum and to absorb nourish- 

 ment from it, 2 and on the other hand there are others whose 

 roots resemble leaves and actually subserve their functions 

 (Dicrcza, Tczniophyllum, etc.). 



The researches on adaptations have taken two direc- 

 tions. In the first place, it was necessary to settle which 

 structural conditions were to be regarded as adaptations, 

 and, in the next place, the question arose : How do adap- 

 tations primarily occur? In other words, there was an 

 effort made to formulate a theory of adaptation. Let us 

 first look more minutely into the facts, and select some of 

 the more important advances, which, however, can only 

 be somewhat briefly discussed. All adaptations are either 

 social, i.e., they refer to the condition of other organisms, 

 or else physical, and are connected with the conditions of 

 life under which nourishment, growth and metabolism take 



1 Cf. Utricularia in An. du jard. bota?u, d. Buitenzorg, vol. ix., and 

 Pflanzenbiol. Schild., ii. 



2 Cf. Zur Biologie, v. Genlisea, Flora, 1893, p. 208. 



