38 THE ELEMENTS OF STRUCTURE. 
remained with the many-sided qualities of Amcebe. Yet 
we must avoid thinking about organs as if they were 
necessarily active in one way only. For many organs, e.g. 
the liver, have several very distinct functions. In addition 
to the main function of an organ, there are often secondary 
functions ; thus the wings of an insect may be respiratory 
as well as locomotor, and part of the food canal of Tunicates 
and Amphioxus is almost wholly subservient to respiration. 
Moreover, in organs which are not very highly specialised, 
it seems as if the component elements retained a consider- 
able degree of individuality, so that in course of time what 
was a secondary function may become the primary one. 
Thus Dohrn, who especially emphasised this idea of 
function change, says: ‘Every function is the resultant of 
several components, of which one is the chief or primary 
function, while the others are subsidiary or secondary. 
The diminution of the chief function and the accession of a 
secondary function changes the total function ; the secondary 
function becomes gradually the chief one; the result is the 
modification of the organ.” The contraction of a muscle is 
always accompanied by electric changes, and in the electric 
organs of fishes we see the electric changes in the modified 
muscular tissue composing the organs becoming more 
important than the contractility. The structure known as 
the allantois is an unimportant bladder in the frog, in Birds 
and Reptiles it forms a foetal membrane (chiefly respiratory) 
around the embryo, and in most Mammals it forms part of 
the placenta which effects vital connection between off- 
spring and mother. ; 
Substitution of organs.—The idea of several changes of 
function in the evolution of an organ, suggests another of 
not less importance which has been emphasised by Kleinen- 
berg. An illustration will explain it. In the early stages 
of all vertebrate embryos, the supporting axial skeleton is 
the notochord,—a rod developed along the dorsal wall of 
the gut. From Fishes onwards, this embryonic axis is 
gradually replaced in development by the vertebral column 
or backbone; the notochord does not become the back- 
bone, but is replaced by it. It is a temporary structure, 
around which the vertebral column is constructed, as a tall 
chimney may be built around an internal scaffolding of 
