658 Geological Periods of Mutation. 
forms of life, have been mutable. Let us consider this 
view in relation to two important results of our investi- 
gation. In the first place it is obvious that Oenothera 
is not the only mutable plant. According to the re- 
searches of BAILEY 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 
assumption 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 mutable main 
lines of the pedigree, 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 freely 
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 
