254 EVOLUTION OF LIFE. 
The Infusoria are not to be distinguished in their early 
stages from Ameeba. Kolpoda-like forms, supposed at 
one time to belong to the Infusoria, have since been shown 
to be the young of Turbellarian Worms. The Infusoria 
seem then to be essentially a transition group; so much so 
that some naturalists have held that the group is not a dis- 
tinct division of the animal kingdom, but simply a collec- 
tion of the young of higher animals. It seems proper to 
mention now the necessity of learning the condition of the 
young, or embryonic stage of animals, whose origin we are 
seeking. Supposing the animal kingdom is really repre- 
sented by a tree, of which the main branches, twigs, and 
leaflets are the orders, families, and species into which 
animals are divided, common features of structure in 
these groups must not be sought at the ends of the 
branches which are far apart, but at the point where the 
branches diverge. To make my meaning clear, take the 
case of young babies, which look very much alike, but 
owing to certain hereditary influences, and the effects of a 
different mode of bringing up, can be readily distinguished 
later in life. The origin of Worms is not to be sought in 
comparing a highly-organized member of the group with 
one of the Infusoria, but by placing side by side a simple 
worm like the Planaria and one of the Animalcula. The 
proofs of the Worms coming from Infusoria are furnished 
by the resemblance of the young of the Soft Worms to 
existing Infusoria,and the peculiarities of structure common 
to both. 
WORMS. 
By looking at Tree III. we see the root of the Worms 
divides into two branches,—the Soft Worms (Scolecida) and 
Sac-worms, —the Soft Worms giving rise to the Articulated 
Worms, in which are seen the beginnings of the Echino- 
dermata and! Articulata, while the Sac-worms are the com- 
