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CHAPTER XXVIII 



WATER PLANTS AND THE 'LAW OF LOSS' 



IN EVOLUTIONi 



IT is a well-known fact indeed almost a truism that 

 structural reduction is one of the most marked charac- 

 teristics of water plants. In the preceding chapters of this book 

 we have alluded to numerous cases in which aquatics are 

 reduced, both in their vegetative and reproductive organs, as 

 compared with their terrestrial relations. The consideration of 

 this reduction, and of some of its sequels,* led the present writer 

 to formulate, under the name of the 'Law of Loss,' a certain 

 minor principle which seems to be operative in various phases 

 of plant evolution. The expression 'Law of Loss' is meant to 

 indicate the general rule that a structure or organ once lost in 

 the course of phylogeny can never be regained; if the organism 

 subsequently has occasion to replace it, it cannot be repro- 

 duced, but must be constructed afresh in some different mode. 

 In the very nature of the case, such a law is not susceptible 

 of formal proof. We can only here consider whether, if accepted 

 as a working hypothesis, it throws any light on structural 

 features observed among water plants, which would otherwise 

 be obscure. We may begin with a case which happened to be 

 the first to arrest the present writer's attention in connexion 

 with the ' Law of Loss,' and to which allusion has already been 

 made^. Ceratophyllum demersum and certain species of Utricu- 

 laria are entirely rootless at all stages of their life-history even 

 the primary root of the seedling being either altogether absent 



1 The greater part of this chapter has already appeared in two papers 

 bv the present writer, Arber, A. (191 8) and (19192), to which reference 

 can be made for a fuller treatment of the subject. The ' Law of Loss ' is 

 closely related to Dollo's ' Law of Irreversibility.' 



2 See pp. 88, 89, 96, 97. 



