CHAP. XV. ON THE DRObERACE^E. 363 



growing in very poor soil, it would tend to be perfected 

 through natural selection. Therefore, any ordinary 

 plant having viscid glands, which occasionally caught 

 insects, might thus be converted under favourable cir- 

 cumstances into a species capable of true digestion. It 

 ceases, therefore, to be any great mystery how several 

 genera of plants, in no way closely related together, 

 have independently acquired this same power. 



As there exist several plants the glands of which 

 cannot, as far as is known, digest animal matter, yet 

 can absorb salts of ammonia and animal fluids, it is 

 probable that this latter power forms the first stage 

 towards that of digestion. It might, however, happen, 

 under certain conditions, that a plant, after having 

 acquired the power of digestion, should degenerate 

 into one capable only of absorbing animal matter in 

 solution, or in a state of decay, or the final products 

 of decay, namely the salts of ammonia. It would appear 

 that this has actually occurred to a partial extent with 

 the leaves of Aldrovanda ; the outer parts of which 

 possess absorbent organs, but no glands fitted for the 

 secretion of any digestive fluid, these being confined 

 to the inner parts. 



Little light can be thrown on the gradual acquire- 

 ment of the third remarkable character possessed by 

 the more highly developed genera of the Droseracese, 

 namely the power of movement when excited. It 

 should, however, be borne in mind that leaves and 

 their homologues, as well as flower-peduncles, have 

 gained this power, in innumerable instances, indepen- 

 dently of inheritance from any common parent form ; 

 for instance, in tendril-bearers and leaf-climbers (i. e. 

 plants with their leaves, petioles and flower-peduncles, 

 &c., modified for prehension) belonging to a large 



