558 Geological Periods of Mutation. 



forms of life, have been mutal)le. Let us consider this 

 view in relation to two important results of our investi- 

 gation. In the first place it is oljvious that Oenothera 

 is not the only mutable phuit. According to the re- 

 searches of Bailev and White tomatoes are now almost 

 certainly undergoing such a change, and cocoa-palms, 

 since their introduction into the Indian Archipelago, must 

 almost certainly have passed through such a period. 

 Everywhere in the vegetable kingdom we come across 

 vestiges of periods of mutation; and we should be led 

 to the conclusion that the phylogeny of plants is repre- 

 sented by a richly-branched pedigree in which, down- 

 wards from the mutational groups now living, the lines 

 must always be composed of mutable ancestors ; for the 

 assum])tion that mutability is an uninterrupted process 

 is exactly the hypothesis from which we start. 



But not all plants and animals are mutable at the 

 present time; on the contrary, mutability is a very rare 

 phenomenon. This circumstance can only be brought 

 into harmony with the theory of the ever mutal)le main 

 lines of the i)edigree, by assuming that they have pro- 

 duced lateral branches in which the capacity for mutation 

 has been lost. That such has often been the case we may 

 confidently infer from the available evidence. Accor- 

 ding to the principles enunciated in the previous chapter, 

 all that is necessary to bring this about, is that the repre- 

 sentative elements be transferred from their unstable 

 into a rigid condition. 



The whole pedigree would then appear as a freel}' 

 branched system of continuous mutable lines without 

 gaps, and which are everywhere clothed, if I may so 

 express it, by numerous immutable lateral branches. 

 These would then stand in the same relation to the stem 



