228 LIFE, in ITS INTERMEDIATE FORMS. 
animals whose structure manifestly ascends in uninter- 
rupted gradation, such as that beautiful chain we noticed 
in the Echinodermata, for instance. It is not that the 
principle is not correct, but that it is not true throughout : 
it is not the only principle. 
In fact, we find, in tracing up linear gradations, points 
occurring ever and anon, where, though the series does 
not break, other series link themselves on laterally, which 
may themselves be pursued linearly, and which in like 
manner send off side-chains, which link with other linear 
series. 
Let us illustrate this by a homely simile. Let us sup- 
pose that a person with eyes capable of only microscopic 
vision had before him a purse made of netted beads, and 
was endeavouring to discover the plan on which it was 
constructed. He is able to see but a single bead at a 
time. He takes one and numbers it ; perhaps names it : 
and is conscious that another is in contact with it ; this 
also he numbers ; and finds a third, and a fourth, and so 
on, running on in straight succession. He presently con- 
cludes that he understands the structure ; it is plainly a 
linear series. But soon he comes to a bead — perhaps it 
is number 10 — which has in contact with it not only the 
number 11, but also a bead at each side. If he leave the 
original line, which ho finds he can follow on if he so 
choose, lie will discover that either of these lateral beads 
is the first of a series, which leads on lineally, till by and 
by, after sundry offshoots and bends, he comes into the 
original line at bead number 20, or 35, or 150, as the 
case may be. And from the offshoots of these secondary 
chains, he can pursue similar chains ever branching off 
