THE PRINCIPLES OF BIOLOGY. 
251 
Central development is distinguished into Unicentral and 
Multicentral, according as the original product develops 
symmetrically round one centre, or without subordination to 
one centre develops in subordination to many centres. The 
Thalassicollie of the southern seas represent among animals 
Unicentral development, while in the realm of plants it is 
exemplified, though feebly, in the Volvox globator. Multi¬ 
central development being far more general is variously 
exemplified in both the animal and the vegetal kingdoms. 
From Central development we pass to that higher system 
of arrangement which Mr. Spencer terms Axial development, 
and this kind of development is of two orders, viz., Uniaxial 
and Multiaxial; examples of both being found in each 
division of the organic world. Multiaxial development 
occurs in all the higher types of vegetal life, while in the 
animal kingdom it prevails only among the lower types, all 
the higher orders being examples of Uniaxial development. 
Both Axial and Central development may be either 
continuous or discontinuous. Among plants continuous 
Multiaxial development is the rule, .and as instances of it 
among animals may be mentioned all the compound 
Hydrozoa and Actinozoa. 
From these general aspects of development we are led to 
the more special aspects. The gradual unfoldment of the 
bud of any plant to the leaf-bearing shoot is a passage from 
the incoherent, indefinite, homogeneous, to the coherent, 
definite, heterogeneous, and the same applies exactly to the 
development of the arm of a man from its bud stage. In 
both cases the original is simple, and having much in common 
with countless other forms, increasing in complexity by slight 
differentiations, they gradually obtain increasing unlikeness to 
other forms. 
In their earliest stages all organisms have the greatest 
number of characters in common with all other organisms 
in their earliest stages, but in later stages the characteristics 
displayed by each structure correspond with a less extensive 
number of organisms ; step by step these resemblances are 
diminished until they are finally narrowed down to the 
members of the same species. Thus while we may not 
believe that man passes through stages which resemble the 
adult forms of lower organisms, we may say that the embryos 
of both man and the lower organisms (the fish for instance) 
present in certain stages characteristics in common. And 
while we cannot say that a man was at one time a fish, or a 
reptile, we may with truth assert that he passes through the 
piscine and the reptilian into the mammalian stage. 
