22S LIFE, IN ITS I XTERMED I ATE 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 he finds he can follow on if he so 

 choose, he will discover that either of these lateral beads 

 is the first of a series, which leads on linearly, 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 



