162 STERILE AND FERTILE REGIONS 
As this in my opinion has not yet been accorded its proper place‘ in the 
evolutionary story, I propose to consider it at some length. 
Imperfectly developed parts have played an important réle in arguments. 
on Evolution. On the Zoological side especially they have been used 
as weighty evidence. Similarly, in Botany they have been the basis of 
discussion: in the morphology of the flower, abortive stamens, carpels, 
pollen-sacs, and ovules have been cited as foundations for elaborate 
argument. Where present in normal position the existence of an abortive 
stamen or staminode has been habitually held to be sufficient indication 
of the previous existence of a fully developed stamen in the ancestral line ; 
and on such evidence natural affinities have been traced and accepted, 
A, median section of young sporogonium ot Axeura ambrosioides. The internal mass 
of cells of the sporcgonial heag, (‘‘archesporium”) is already differentiated so as to 
indicate the sterile elaterophore, and the outer fertile region. JZ, the same, older: the 
indications of sterilisation have extended outwards, and it is only the peripheral fringe of 
cells (shaded) which will be sporogenous. C, transverse section of the same. X 150. 
usually without question. But floral morphology has gone further: com- 
parative study has led to the conclusion that in certain ancestral lines 
of descent parts have existed, which in the individuals of the present day 
are entirely unrepresented by any vestigial growth. This condition of 
complete disappearance of a part or parts has been styled “ablast,” as 
distinct from “abortion,” where the incompletely developed part has an 
objective existence. Eichler maintained that the conditions distinguished 
as “abort” and “ablast” are not essentially different in kind, but only 
differ in degree. He points out that abortion itself is not susceptible of 
objective proof, and it may be remarked incidentally that it is this fact 
which has prevented the full recognition of the part which it has played 
in the origin of the sporophyte. Speaking of the relations between partial 
abortion, where a vestigial structure is present [‘abort”], and complete 
suppression [“‘ablast”], Eichler remarks (Bévthendiagramme, p. 52) that in 
